Å triple images suggest negligible heating from the smallest campfires and
most from the largest CBP reconnection events. [[GoBack Button: Back]] [Main
call] the shortest, but AIA 193 Å movies show frequent change suggesting
continuous renewal. The representative AIA 193 Å images in the triple blinkers
suggest that CBPs are the main agent in quiet-Sun coronal heating and cause
the diffuse AIA 193 Å brightness spread widely around them, together and with
contributions from prior CBPs. Such diffuse brightness is absent in coronal
holes where heating does not spread to surrounding surface areas along closed
loops but instead follows open-field up and out, likely as coronal plumes
(Wang et al. 1997).

To reproduce such dynamic nature numerically, even for just “quiet”
chromosphere, poses enormous challenges because the MHD simulations must
produce spicules-II as copiously as actual solar network does and must then be
combined with 3D(t) NLS–NLW–NLT (“NSE”) spectral synthesis including PRD at
least for Ly$\hskip 0.86108pt\alpha$ to recover the observed H $\alpha$
chromosphere and in addition must produce small-scale bipolar activity causing
and governing CBPs as observed.

ALMA has the potentiality to sample and measure the quiet chromosphere sharper
and faster than DKIST [80 quiet chromosphere with ALMA]8080endnotetext: quiet
chromosphere with ALMA
Outside rare truly quiet internetwork (endnote 6) ALMA samples chromospheric
canopies that are also but differently sampled by H $\alpha$ and Ly$\hskip
0.86108pt\alpha$ (Rutten 2017b). The IBIS H $\alpha$ core-width (pdf 8) scene
represents an analogon of an ALMA active region scene (but mapping rather then
measuring temperature). The SST H $\alpha$ core-width scene in the final
showex display of my alignment practical samples a quiet disk-center coronal
hole and represents a quietest-Sun ALMA analogon. Claiming that longer ALMA
wavelengths sample higher layers from the $\lambda^{2}$ increase in
$\alpha_{\lambda}^{\rm ff}\\!\sim\\!\\!N_{\rm e}\,N_{\rm
p}\,T^{-3/2}\lambda^{2}$ [RTSA Eq. 2.79 (pdf 47)] is as plane-parallel wrong
as interpreting H $\alpha$ images with the famous VALIIIC diagram [SSX (pdf
60)] (Sect. 6). The quiet chromosphere is not layered but dynamically
structured with NSE hydrogen-top opacities (endnote 8).. ALMA brightness does
measure temperature for optically thick features but the feature proton
densities and hence the $\propto\\!\lambda^{2}$ feature opacities and hence
the $\tau\\!=\\!\\!1$ feature sampling locations were set NLT-wise in the
recent past. Some features may be thin at 1 mm and thick at 3 mm, others so
thick that the measured temperatures sample adjacent outer feature surfaces,
further out at longer wavelength. The H $\alpha$ opacities are also set NLT-
wise but H $\alpha$ and Ly$\hskip 0.86108pt\alpha$ obtain relative feature
contrasts from variations in $\sqrt{\varepsilon}$ scattering with deep
$\tau_{\rm eff}\\!=\\!\\!1$ signature and from Dopplershifts. Studying the
nature and formation of canopy features requires follow-the-motion along-the-
feature tracking with temporal delay comparisons. Frequency-resolving Fourier
power, phase difference and coherency analysis should also be along-the-
feature. ALMA already excels in obtaining fast cadence and potentially excels
in obtaining angular resolution117117117Addition of many longer baselines may
improve ALMA’s sharpness beyond SST/CRISP H $\alpha$ scenes, also beyond yet
sharper SST/CHROMIS H $\beta$, hopefully beyond DKIST. Asymmetric
$(U,V)$-plane filling yielding anisotropic resolution is an option since the
solar scenes consist of long thin fibrils spread in all directions, so that
high resolution only transverse to or along suited ones is already worthwhile
(as with the 11-m GISOT design of Hammerschlag et al. (2004) with an elliptic
aperture (Fig. 1a (pdf 2)) to minimize prime-focus heat load). while spectral
profile resolution in Balmer and Lyman lines offers larger opacity diversity
and Dopplershift encoding; combination of these diagnostics and/or less
saturated companions (footnote 128) is desirable. [[GoBack Button: Back]]
[Main call] [81 ALMA – H $\alpha$ – SDO alignment]8181endnotetext: ALMA – H
$\alpha$ – SDO alignment
Another confirmation that ALMA sees chromospheric canopies in quiet-Sun areas
is that such ALMA images can be co-aligned with H $\alpha$ images through
cross-correlation. This is easy on the disk with co-pointed H $\alpha$ imaging
spectroscopy delivering temperature-mapping H $\alpha$ core width (also shown
in the IBIS blinker of endnote 8). The correspondence is not 1:1 because hot
RBE and RRE tips, emanating furthest from the spicule-II launching network,
are not seen in H $\alpha$ core-width imaging which mostly charts cooling
return fibrils. Blurring may improve pattern correlation. Near the limb ALMA
emphasizes hot tips of spicules-II that appear as upright hedges of straws,
bright in Ca II H & K (DOT Ca II H movie, Fig. 1 (pdf 2)) of Rutten 2006),
dark in the H $\alpha$ red and blue wings (Fig. 5 (pdf 5) of De Pontieu et al.
2012, Fig. 1 (pdf 4) of Rutten 2013) and with bright feet in IRIS 1400 Å
slitjaw images (Fig. 1 (pdf 2) of Yokoyama et al. (2018)). Hence, near-limb
ALMA images may be cross-correlated to underneath summed and reversed H
$\alpha$ blue-wing and red wing intensity. Off the limb non-reversed H
$\alpha$ core intensity of sufficient resolution may serve for tall spicules-
II jutting out from the spicule-I forest. The first co-pointed ALMA – H
$\alpha$ data were with DST/IBIS (Molnar et al. 2019) but IBIS has left the
DST for which new Fabry-Pérot instrumentation is planned. The DST is located
at 2.5 hours later longitude than ALMA. The SST on La Palma is at 3.3 hours
earlier, the GST at Big Bear 3.3 hours later, the DKIST on Maui 5.9 hours
later. Afternoon seeing can be good at La Palma after the Sun has passed the
Caldera while the USA sites usually have best seeing soon after sunrise. An
attractive alternative is to move the relocatable and currently mothballed DOT
from La Palma to ALMA. It anyhow has to make room for the planned EST and it
packs into a few containers. On the disk two-channel speckle bursts in 3 or 5
H $\alpha$ wavelengths with its tunable Lyot filter would deliver qualitative
core width mapping, simultaneous granulation images giving co-registration
with SDO/HMI and other SDOs with my alignment pipeline (endnote 8). Inside the
limb summed and reversed H $\alpha$ wings would do, off-limb non-reversed H
$\alpha$ core summation, both with Ca II H to AIA 1700 Å outer-disk
coalignment. Continuous co-pointing may then often enable precise co-alignment
of solar ALMA data with all SDO diagnostics. [[GoBack Button: Back]] [Main
call] [82 ALMA – GONG – SDO alignment]8282endnotetext: ALMA – GONG – SDO
alignment
In the absence of co-pointed H $\alpha$ spectrometry ALMA co-alignment may be
tried for quiet on-disk scenes with wider-band H $\alpha$ core images
exhibiting cooling return fibrils that remain dark through non-SE overopacity.
With greyscale reversal rough pattern agreement with the fibril temperatures
mapped by ALMA enables co-alignment, easiest for large quiet fields. This
correspondence is not 1:1 also temporally because the cooling return fibrils
follow a few minutes after the hottest spicule-II tips (Rutten et al. 2019).
Space-time blurring and delayed H $\alpha$ sampling may improve pattern
matching. Such images are almost always available from GONG118118118With large
quality variations with time and between stations. Fortunately ALMA’s
longitude picks the usually better ones (Big Bear and Cerro Tololo).. Since
these are full-disk they may contain a suited large quiet closed-field area
somewhere for precise co-alignment (with H $\alpha$ intensity reversed) to
simultaneous AIA 304 Å images as in the blink for the first SolO/EUI target in
Figs. 11 – 12 (pdf 29) of LAR-1 and discussed in Appendix B (pdf 11) there.
The GONG H $\alpha$ part (also greyscale-reversed) covering the small ALMA
target field may then serve as ALMA-to-SDO intermediary for wherever ALMA
pointed. I succeeded in this for quiet-Sun ALMA data with a good GONG
sequence, found the second-step ALMA-to-GONG matching easier than the first-
step GONG-to-304 Å matching, and indeed found good pattern correlation between
the ALMA, GONG and AIA chromospheres. This suggests that many ALMA quiet-Sun
on-disk sequences can be precisely coaligned with all SDO diagnostics via
GONG. For active regions the partial match between the IBIS H $\alpha$ core-
width scene (pdf 8) and AIA 304 Å (pdf 10) in the IBIS – SDO active region
blinker of endnote 8 suggests that direct ALMA to 304 Å alignment maybe the
best to try but it will be severely hampered by small ALMA field size and the
AIA 304 Å deterioration (footnote 120). [[GoBack Button: Back]] [Main call].

### Active chromosphere.

For chromospheric activity phenomena I refer to Jongchul Chae’s contribution
in this school because I have not worked on these except EBs and FAFs [83
Ellerman bombs and flaring active-region fibrils]8383endnotetext: Ellerman
bombs and flaring active-region fibrils
EBs are active-region MC reconnection events in the low photosphere. After
growing interest in the past decade119119119I participated in Watanabe et al.
(2011), Rutten et al. (2013), Vissers et al. (2013), Rutten et al. (2015),
Vissers et al. (2015), Rutten (2016), Young et al. (2018), Vissers et al.
(2019) and think that I helped boost interest in Ellerman (1917): after half a
century of disregard it shot up in ADS citations reaching 100 in its
centennial year. Moral: if your nice discovery goes unnoticed relax waiting
for ADS to revive it. More moral: this revival got my laptop confiscated by
border police spotting the b* word and taking months to classify me as
harmless Ellerman b*er – hence using b=burst in Young et al. (2018) – but note
that a solar EB releasing 20 Gt (Rezaei and Beck 2015) is a bigger blast than
the champion terrestrial 50 Mt “Tsar Bomba” hydrogen b*. and successful
simulation by Hansteen et al. (2017) and Ortiz et al. (2020) they now
represent another H $\alpha$ feature that is largely understood. Here they are
of chromospheric interest because the spectral signature of these events is
complex and instructive. EBs appear bright only in the H $\alpha$ wings
because their H $\alpha$ line cores remain shielded by overlying dark active-
region fibrils (Fig. 3 (pdf 4) of Rutten et al. 2013). They are therefore
similarly shielded and invisible for ALMA (prediction 9 (pdf 9) of Rutten
(2017b) confirmed by da Silva Santos et al. 2020). EBs are not visible in the
Na I D and Mg I b lines (Ellerman 1917, Rutten et al. 2015) because they are
hot enough to fully ionize neutral metals. Similarly EBs appear bright in UV
continua from Balmer continuum brightening with neutral-atom bound-free
opacity ionized away (as in flare feet showing optical flares). Vissers et al.
(2019) have defined and calibrated an EB detector using AIA 1700 Å images
which opens the entire SDO database for studying EB occurrence. In AIA 1600 Å
images EBs gain additional brightness from the C IV lines in the passband,
similarly to the large Si IV enhancements in Fig. 4 (pdf 7) and Fig. 8 (pdf 9)
of Vissers et al. (2015). The latter figure also shows Mn I, Fe II and Ni II
absorption blends on high Si IV, C II and Mg II emission peaks. Figure 2 (pdf
5) of Rutten (2016) added a schematic EB into the last panel of Fig. 1 of the
simulation of Leenaarts et al. (2007) to show how these can arise from
undisturbed EB surroundings, not part of the phenomenon as suggested for IRIS
bombs by Peter et al. (2014). Because EBs are hotter and denser than
internetwork and dynamic-fibril shocks in which Ly$\hskip 0.86108pt\alpha$
already reaches collisional equilibrium their H $\alpha$ extinction obeys SB
partitioning, permitting the straightforward SB opacity comparisons in Fig. 5
(pdf 9) of Rutten (2016). In this figure H $\alpha$ reaches similar extinction
maxima as the Mg II and Si IV IRIS lines, so that the actual EB H $\alpha$
profile under the canopy is probably similar to Mg II k with a high peak and a
small central dip as in Fig. 4 (pdf 7) of Vissers et al. (2015), hence with
much larger profile-summed brightness than observed. It is a mistake to count
the observed H $\alpha$ EB wing enhancements as contribution to coronal
heating since this energy is radiated away (endnote 3) while the H $\alpha$
core photons are boxed in by overlying NLT-opaque canopies but appear
insufficient to brighten these. Small hot photospheric structures as EBs
containing intense Ly$\hskip 0.86108pt\alpha$ irradiate their cool
surroundings causing aureole boosting of the hydrogen top including ionization
[SSX (pdf 171)] and making the feature visible even if the actual reconnection
site is too small. NSE recombination is slow so that such boosts linger
longer, in a wake if the reconnection progresses upward. The Zanstra mechanism
for detour photon conversion in planetary nebulae [SSX (pdf 144)] [IART Sect.
8.3.3.1 (pdf 137)] likely also operates in their cool surroundings. Presently
EB interest has moved to non-active quiet-Sun QSEBs (“quiet-Sun Ellerman-like
brightening”) marking ubiquitous but hard-to-detect small-scale photospheric
reconnection (Rouppe van der Voort et al. 2016, Danilovic 2017) and best seen
in SST/CHROMIS H $\beta$(Joshi et al. 2020). FAFs were called “transient
loops” by Pariat et al. (2009). Vissers et al. (2015) used “flaring arch
filaments” but I renamed them to “flaring active-region fibril” in Rutten
(2016) to avoid confusion with arch filament systems. In active regions with
much flux emergence they appear in AIA 1600 Å image sequences with shorter
duration and more abrupt changes than comparatively stable and roundish EBs,
have elongated morphology, and show fast apparent brightness motion along
filamentary strands. Further description including EB and FAF distinction in
Appendix A (pdf 9) of LAR-1. [[GoBack Button: Back]] [Main call]. Hence no
discussion of spot chromospheres, flares, filaments, etc. here. I restrict
this section to two exemplary active region scenes in different diagnostics.

The first scene is the DST/IBIS – SDO multi-wavelength image mosaic of Cauzzi
and Reardon (2012) and Reardon and Cauzzi (2012a, 2012b) showing AR 1092 near
disk center on 2010-08-03 in the early days of SDO120120120The deterioration
of AIA sensitivities, worst for chromosphere and prominence telltale 304 Å,
during the immensely productive – bread and butter for very many colleagues –
first SDO decade makes me long for AIA-v2 – with yet more pixels, yet more
wavelengths, yet faster?. I received these concurrent co-aligned images in
2011 from K.P. Reardon and combined them into this pdf blinker121121121In a
sequence suited to pairwise page blinking. Most pdf viewers furnish blinking
by flipping pages in-place with the cursor arrow or page up-down keys. Full-
page, fit-to-page, presentation-mode viewing may help. Multiple-page jumping
may be possible with a back key. Myself I instead use my showex blinker from
the command line (script in my SDO manual) because it auto-blinks between
selectable pairs at selectable speed and offers zoom-in magnification with
scatter-diagram correlation and more.. The IBIS selection samples Ca II 8542
Å, H $\alpha$ (also H $\alpha$ core width and core-minimum Dopplershift as
defined in Cauzzi et al. 2009), and the (small) equivalent width of He I D3.
The SDO selection adds comparable HMI and AIA diagnostics.

This IBIS–SDO blinker represents an active-region counterpart to the final
showex inspection of the quiet-Sun SST and SDO sequences in my alignment
practical. These two solar displays are utterly different. The SST set is a
time sequence of a small quiet coronal-hole field with only some network. The
IBIS set is only a single snapshot, not permitting time-delay cause-and-effect
sequence inspection, but shows a larger active-region field containing dense
unipolar plage, mossy plage [84 moss]8484endnotetext: moss
“Spongy” active-region plage showing many short loops and bright footpoints
interspersed with darker patches at small scales in 171 Å and other EUV lines
(e.g., Berger et al. 1999b, 1999a, de Pontieu et al. 1999, Martens et al.
2000, De Pontieu et al. 2003). Polito et al. (2020) differentiate moss and
outflow regions in active-region plage where the latter likely contribute to
the slow wind and non-CME FIP generation (Fig. 8.8 (pdf 170) of Reames 2021;
endnote 8). [[GoBack Button: Back]] [Main call], a large regular spot, active-
region filaments, and even a small flare originating in the spot which behaves
as a CBP with loop brightening and feet heating. Blinking these images
represents a vivid course in active-region phenomenology and also exhibits
aspects discussed above [85 IBIS–SDO active-region blinker]8585endnotetext:
IBIS–SDO active-region blinker
Download here. I made it in 2011 while teaching this course at LMSAL (Palo
Alto) and displayed it ever since in my teaching escapades. Most of these
images have enhanced grey scales122122122Real scientists do not fake-color
intensity scenes. by bytescaling the square root or the square of the measured
intensity plus clipping. All have a small area near center magnified four
times in the upper-right corner for more detailed blinking. Here are my
inventory and tentative interpretations (better blink yourself! – how in
footnote 121): HMI Stokes $I$ – $V$ (p1 – p2): Stokes $I$ shows only granules,
pores and the spot, not the network and plage seen in Stokes $V$ (higher
resolution would show abnormal granulation in plage). Plage brightens in such
continuum sampling only near the limb, from MC transparency in slanted viewing
(righthand cartoon in Fig. 73 (pdf 91) of LAR-1). The flare is along
connections from the spot to opposite polarity in the sunspot moat (blink p2 –
p6). HMI Stokes $V$ – AIA 1700 Å (p2 – p3): ultraviolet MC brightening is not
from higher-up heating (as commonly thought) but from magnetic hole deepening
(lefthand cartoon and caption of Fig. 73 (pdf 91) of LAR-1). Blinking these
disk-center images shows co-spatiality but towards the limb 1700 Å bright
points shift relatively limbward, 1600 Å yet more, also not due to offset in
formation height but to increasing MC transparency (endnote 6). A few bright
1700 Å points in internetwork mark isolated MCs (footnote 8). Elsewhere the
grey fine structure in 1700 Å internetwork areas represents clapotispheric
shocks (endnote 6). AIA 1700 Å – Ca II 8542 Å wing $I$ (p3 – p4): in the Ca II
8542 Å wing MC brightening in plage and network is hole brightening from less
collisional damping and additional downdrafts, but the inset suggests much
shielding within plage. Enhanced bright points in the sunspot moat are likely
not EBs since they lack corresponding excess 1700 Å brightness (Vissers et al.
2019) and may mark MC downdrafts common in moats (Beck et al. 2007).
Internetwork areas show predominant reversed granulation (inset) and also
sporadic shock brightening in grains (Sect. 5.5 (pdf 13) of Rutten et al.
2011). Slender dark fibrils extending from plage and network represent the
plage counterpart of network RBEs combining blueshift and line broadening in
this blue-wing sampling as in Fig. 1 (pdf 3) of Rouppe van der Voort et al.
(2009). Ca II 8542 Å wing $I$ – core $I$ (p4 – p5): dramatic difference. The
core shows the chromosphere with bright-grained network and plage hearts
signifying onset of heating. Magnified blinking shows that many of these dense
bright grains are roughly cospatial with wing MCs, suggesting upward heating,
mostly in unipolar plage (p2). Blinking to 171 Å (p11) shows a few areas
appearing as EUV moss (endnote 8) over bipolar plage (one patch is above the
cutout square but HMI does not show all opposite-polarity field). Dark fibrils
extend far over internetwork but do not cover all of that, some internetwork
still shows up in the quietest areas with shock brightening in grains of which
some are also visible in the wing image. Ca II 8542 Å core $I$ – H $\alpha$
core $I$ (p5 – p6): similar scenes but all fibrils are much fatter and longer
in H $\alpha$ which I attribute to large NSE opacity (endnote 8). All
internetwork is covered by fibrils in H $\alpha$. The bright plage grains
often correspond well between the two (inset). Some may be heated tips of
dynamic fibrils observed down-the-throat. There are long slender dark active-
region filaments above polarity inversion lines (blink either against p2). The
spot flare shows its concurrent loop and feet brightening similarly in both
line cores. H $\alpha$ core $I$ – minimum shift (p6 – p7): the second is the
shift of the H $\alpha$ profile minimum per pixel following Cauzzi et al.
(2009), with blueshift dark. The hearts of the plage areas have small-scale
grainy appearance with sizable patches of large blueshift. The fibrils
extending away show lower-amplitude fibril-to-fibril modulation, a signature
of their intermittent repetitivity in extending out and shrinking back (called
“breathing” by a colleague). Blinking the inset shows short dark blueshifted
heating onsets; the longer grey fibrils are return flows. The latter show
similar but finer structure in Ca II 8542 Å $I$ (blink p5 – p7); showex zoom-
in shows two-branch scatter diagrams there because Doppler-shifting the narrow
Ca II 8542 Å core brightens its intensity for both blue- and redshift. In H
$\alpha$ the umbra shows flashes on their way to running penumbral waves. Only
the brightest are visible in Ca II 8542 Å but they are well-known from Ca II H
& K as in Figs. 2 – 4 (pdf 4) of Rouppe van der Voort et al. (2003). The spot
flare shows a bright core-shift feature in the loop and also a dark shift
companion feature further out, at image edge. Diametrically opposite on the
other side of the spot there is a conspicuous bright-dark feature pair that
also seems connected. H $\alpha$ core width (p8): width of the H $\alpha$ core
per pixel regardless of Dopplershift following Cauzzi et al. (2009) (endote
8). This is the most telltale scene in this blinker with respect to heating.
The plage hearts are grainy and mostly hot. The bushes extending away from
there are shorter than the fibrils in core intensity (p6) because these are
RBE-type heating onsets. Network RBEs reach much further out while heating and
accelerating, but their profile signature moves out from the core to the far
wing (Fig. 5 (pdf 6) of Rouppe van der Voort et al. 2009). The same likely
happens for these active-plage fibrils, as for the active-network fibrils in
the data of Rutten and Rouppe van der Voort (2017) analyzed in Rutten et al.
(2019). Blinking the inset against minimum shift (p7) confirms that the short
dark blueshifted features there are part of these. The active-region filaments
appear about as wide as in He II 304 Å, wider than in H $\alpha$ core $I$ and
Ca II 8542 Å core $I$ which are affected by Dopplershifts. The Dopplershift
features of the spot flare show heating here, also on the other side of the
spot. He I D3 – He II 304 Å (p9 – p10): the He I D3 equivalent-width image is
naturally noisy but nevertheless intriguing. The two scenes show fair bright-
bright correspondence, better further away from the spot and not 1:1. The
proposed mechanism that He I D3 obtains opacity by He I photoionization from
irradiation by hotter overlying gas, often invoked to explain coronal-hole
visibility in optical He I lines (particularly in more opaque He I 10830 Å),
is therefore not the whole story. Furthermore, this blink shows significant
feature shifts, also in the inset, for the bright beginnings of dark fibrils;
they occur further out from network in 304 Å whereas the He I D3-opaquest
(bright) parts remain co-located with corresponding H $\alpha$ features. These
offsets suggest outwardly increasing heating as occurs for network RBEs in the
bottom panels of fig. 6 (pdf 8) of Sekse et al. (2012), here for fatter
fibrils originating from plage. This blink also indicates that the 304 Å dark
filaments are also present in He I D3, barely outlined by showing slightly
less noise. 304 Å (p10): blinking against H $\alpha$ core $I$ (p6), H $\alpha$
core Doppler (p7), H $\alpha$ core width (p8) and He I D3 equivalent width
(p9) are all interesting. The first (p10 – p6) shows coarse similarity between
H $\alpha$ and 304 Å: active chromosphere roughly corresponds between these
diagnostics, opposite in sign to the reversed-H $\alpha$ – 304 Å correlation
for quiet closed-field chromosphere (endnote 8). The correlation improves with
spatial and temporal averaging (options in showex). I have used this likeness
for direct co-alignment of H $\alpha$ to AIA 304 Å for SST targets with much
activity and many active-region filaments and it became an option in my SDO
pipeline (endnote 8), but usually alignment of quiet-area AIA 304 Å to HMI
magnetograms plus SST alignment to HMI per granulation works better. The
second (p10 – p7) blink shows that the brightest 304 Å patches and
correspondingly the brightest He I D3 opacity patches match best with the
plage hearts where H $\alpha$ Doppler is fine-scale black-and-white speckled
indicating upward heating (endnote 8). 171 Å (p11): obviously mostly corona,
no chromosphere. Blinking to HMI Stokes $I$ (p1) shows that all long loops
connect spot areas. The mossy patches near image center correspond to bright
areas in both He diagnostics (p9 and p10); in H $\alpha$ (p6) these may harbor
dynamic fibrils but their identification needs a time sequence. Otherwise, the
grainy areas of chromospheric plage heating in H $\alpha$ width (p8) show no
coronal counterpart. The separation between chromospheric and coronal heating
in quiet closed-field areas is already striking (endnote 8); here it extends
to an active region. Some thick 304 Å filament parts are also dark in 171 Å,
suggesting bound-free scattering as in endnote 3. [[GoBack Button: Back]]
[Main call].

One striking difference between quiet and active scenes is that quiet closed-
field areas permit H $\alpha$ and He II 304 Å co-alignment by reversing H
$\alpha$ intensity (endnote 8, Appendix B (pdf 11) of LAR-1) whereas active
areas permit co-alignment using normal H $\alpha$ intensity (blinker p6 –
p10). I attribute this difference to different ratios between roundabout
(“around”) chromosphere heating that dominates in quiet closed-field areas and
upward chromosphere heating that dominates in the hearts of active network and
plage from more numerous and denser MC clustering [86 around and upward
heating]8686endnotetext: around and upward heating
In quiet areas dark fibrilar H $\alpha$ canopies and similar-pattern but
bright He II 304 Å canopies are made by spicules-II extending from network
(endnote 8). The active region in the IBIS blinker shows similar but denser,
fatter, longer H $\alpha$ fibril canopies (pdf 6) that are relatively dark in
304 Å (pdf 10). It seems likely that such active canopies stem from similar
mechanisms as quiet canopies; I call these “around heating”. However, the
hearts of plage and network show dense grainy heating in H $\alpha$ core width
(pdf 8), especially where mostly unipolar, that I call “upward heating” –
meaning at chromospheric heights, not high-reaching as coronal plumes in
coronal holes as in Wang et al. (1997). This heating follows fields that do
eventually bend back to the surface but connect along long coronal loops to
faraway activity. H $\alpha$ core-width (pdf 8) shows remarkable difference:
grainy bright upward heating in the plage hearts versus around heating as
extended bushes pointing away from network and plage that seem comparable to
spicule-II onsets (endnote 8). I attribute this difference to MC clustering
and upward field forcing. Active network and plage contain discrete MCs in
larger number and at larger density than quiet network. In classic
magnetostatic fluxtube cartoons [SSX (pdf 44)] the field flares out with
height; in classic “Zürich wineglass” models (Fig. 4 (pdf 6) of Bünte et al.
1993) neighboring fluxtubes inhibit flaring wide. Denser near-unipolar
collections so have more upward field, as in the simulation-assisted inversion
map in Fig. 7 (pdf 9) of Jafarzadeh et al. (2017). I think that the resulting
dominance of upward heating over around heating causes larger brightness in
304 Å, outshining the surrounding fibril canopies that then appear dark in
byte-scaled comparison. Where plage and network are more bipolar they have
small-scale short-loop connectivity leading to moss appearance (endnote 8).
For both around and upward heating the driving mechanisms are not known, as is
the case for spicules-II producing canopies around quiet network in closed-
field areas (fewer in coronal holes, e.g., Fig. 42 (pdf 60) of LAR-1).
However, also for quiet canopies there is observational inclination dichotomy
between more horizontal and more vertical. On-disk RBEs and RREs are easiest
detected in the H $\alpha$ wings when they reach far out from their launching
network, therefore preferentially showing wide-spread around heating, but at
the limb spicules-II are easiest noted and isolated amid their projective
confusion when they are more upward, as shown in the inclination histograms in
Fig. 4 (pdf 5) of Pereira et al. (2012) and seen near the limb e.g., in this
DOT straw movie and in Fig. 5 (pdf 5) of De Pontieu et al. (2012). Hence
distinction between around versus upward morphology and heating seems gradual,
suggesting similar driving but difference in appearance and detectability set
by variation in dominating field topography defined by MC clustering density
and polarity. The recent CLASP2 – IRIS co-observation of Ishikawa et al.
(2021) indicates that fields keep longitudinal strength up to 300 Gauss high
in the chromospheric above dense plage, suggesting preponderance of straight-
up fields. The precise pattern correlation between field strength and Mg II k
core intensity and the good correspondence with AIA He II 304 Å intensity in
their Fig. S1 (pdf 44) confirm straight-up heating. For He I D3 the sparse and
minor on-disk presence in the IBIS blinker (pdf 9) versus its bright Balmer-
like prominence in the flash spectrum, also away from active regions, suggests
sensitive response to upward network and plage heating (endnote 8). [[GoBack
Button: Back]] [Main call]. I also wonder whether or how upward chromosphere
heating in the hearts of active network and plage contributes to the slow-wind
FIP composition bias [87 FIP effect]8787endnotetext: FIP effect
About 3$\times$ relative overabundances compared to photospheric values in the
slow solar wind (SSW) and in solar energetic particles (SEP) for elements with
low neutral-atom ionization “potential” (FIP) (e.g., Meyer 1985, 1991, Fig. 7
(pdf 11) of von Steiger and Geiss 1989, Fig. 8.5 (pdf 164) of Reames 2021).
These include the most abundant electron-donor elements Fe, Si, Mg, Al [SSF
(pdf 56)] which are predominantly ionized everywhere but amount to only
$10^{-4}\,N_{\rm H}$ in the photosphere where H and hence the gas are near-
neutral. This “FIP effect” is generally attributed to ion-neutral segregation
involving the degree of line-tying to open field and hence attributed to low-
atmosphere origins (e.g., von Steiger and Geiss 1989, cf. Laming et al. 2019,
Reames 2021). Modeling thus must include neutral-ion separation already at low
heights. The upper panel of Reames’ Fig. 8.5 (pdf 164) indicates pivots near
10 eV for the SSW values, 14 eV for the SEP values. This split suggests that
for the SSW the 10.2 eV Ly$\hskip 0.86108pt\alpha$ jump is the key in hydrogen
ionization by controlling the feeding level of the Balmer ionization-
recombination loop and so defining NSE retardance of the hydrogen-top (high
levels and ion) populations in post-hot cooling gas, as described for shocks
and spicules-II in Sects. 7 and  8. Presence or absence of frequent hot-cool
cycling may be important. The SEP pivot instead suggests hydrogen ionization
directly from the ground level, hence in more abrupt and fiercer heating
events. The IRIS study of Polito et al. (2020) points to non-mossy upward
active-region heating (endnote 8) in slow-wind generation. The match in the
IBIS blinker of “brightest” plage-heart patches in He I D3 opacity with
grainiest H $\alpha$ core Dopplershift patches in plage (p7 versus p9 of the
blinker) marking upward heating suggests to test the He I lines (endnote 8) as
potential proxies for locating upward-heating FIP-offset source areas,
possibly thanks to similar retardance boosting. [[GoBack Button: Back]] [Main
call].

The enigmatic appearance of the chromosphere in optical He I lines may also
have mostly to do with upward heating [88 optical He I lines]8888endnotetext:
optical He I lines
The formation of optical He I lines remains an outstanding enigma. The main
ones are He I 10830 Å, weaker He I D3 at 5876 Å, and He I 6678 Å “occasionally
making its appearance” (Lockyer 1868). He I 10830 Å reaches largest extinction
because its lower level is metastable and collects population. Its observed
depth is claimed as coronal activity mapper in the optical (e.g., Fig. 1 (pdf
3) of Penn et al. 1998) – but blinking He I D3 against AIA EUV in the IBIS
blinker shows better He I matching with chromospheric 304 Å than with coronal
171 Å. The line serves as coronal hole mapper in the McIntosh Archive (e.g.,
Webb et al. 2017), perhaps better than coronal activity mapping since the AIA
fire detectors (endnote 8) show less heated quiet chromosphere in holes, hence
less He II and He I-top population. Lockyer proposed element “helios” for He I
D3 which became helium and remains the only extra-terrestrially discovered
element since coronium ([SSX (pdf 8)]) and nebulium ([SSX (pdf 142)] ff) went
away. I am not aware of a sound explanation why his He I D3 is so
extraordinary strong off-limb, equal to H $\alpha$ in his engraved Fig 42 (pdf
95) and ranking between H $\gamma$ and H $\beta$ in brightness and off-limb
extent in pages 94–95 of Table 3A (pdf 94) of Dunn et al. (1968), much
brighter and higher than its Na I D neighbors whereas it is nearly invisible
on the disk and hence not even mentioned on page 258 (pdf 292) of the line
table of Moore et al. (1966) from the Utrecht Atlas of Minnaert et al. (1940)
in which the Na I D lines are the darkest in the optical and the Balmer lines
are prominent wide dips (Fig. 10 (pdf 18) of Rutten 2019). Similarly, He I
10830 Å is absent in the ALC7 spectrum [SSX (pdf 94)].123123123Imagine solar
physics when the Balmer and Ca II lines would similarly be nearly absent on
the disk while still bright out to champion height in the flash spectrum. We
would still have Lockyer’s chromosphere, less push for Lyot and Fabry-Pérot
filters, longer longevity of optical Fourier spectrometry. The 1D standard
models would still have their near-104 K plateaus but correctly attributed to
shock heating (“clapotisphere” instead of “chromosphere”). Without blanketing
canopies H $\alpha$ would show these shocks in its core but less informative
than Ca II H & K; ALMA would sample shock temperatures in all bands. The main
on-disk indication of magnetic chromosphere heating outside active regions
would be the bright patches around magnetic network in He II 304 Å images,
correctly attributed to the taller spicules observed off-limb in H & K and the
Balmer lines. How can the Balmer lines in the top of the hydrogen atom and the
He I lines in the top of the helium atom differ so enormously between
transverse and top-down viewing? 124124124The 400+-page book by Athay and
Thomas (1961) (wrong order at ADS) addressed this question without clear
answer. It was grandly titled “Physics of the solar chromosphere” earning a
scathing review from De Jager ([SSX (pdf 15)] ff) for not mentioning magnetism
nor dynamism. De Jager added that Thomas’ “photoelectric control” producing
$S>B$ was the main item of interest but even that was misleading (endnote 3).
Off-limb (no continuum) large emissivity can go with small extinction through
large $\varepsilon B$ and/or large $\eta S^{\rm d}$ contributions in Eq. 3.
Because He I D3 shows Balmer-like extent above the limb while the Balmer
extent is dominated by quiet-Sun spicules-II and He II 304 Å shows quiet-
chromosphere canopies pattern-matching the spicule-II-made reversed-H $\alpha$
ones outside coronal holes (endnote 8) I suggest that in quiet-Sun around
heating (endnote 8) the He I top shares in the spicule-II emissivity phase
giving large $\varepsilon B$ off-limb brightness, but less in the subsequent
recombination phase giving the H I top sufficient NSE extinction to show
opaque H $\alpha$ canopies on the disk with $\sqrt{\varepsilon}$ darkness and
in ALMA with temperature-defined brightness. Small top-down extinction in
resolved on-disk features can go with large off-limb emissivity from summing
many features along the tangential line of sight.125125125In this
interpretation the opaque fibrilar chromosphere covering most of the disk in H
$\alpha$ and ALMA is the non-SE weirdo, rather then its absence in He I lines;
one shouldn’t study this over-opaque chromosphere in wondering what heats the
corona. Spicule-II RBEs and RREs and return fibrils in H $\alpha$ occur
ubiquitously also in coronal holes but these show less heated 304 Å
chromosphere126126126High-resolution showcase: blink the AIA fire detector and
SST H $\alpha$ core-width images in my alignment practical.. Maybe He I
outside coronal holes remains considerably photoionized by blanket-like EUV
active-corona irradiation from wide-around above so that the He I and He II
304 Å lines map coronal holes by their absence while H $\alpha$ shows on-disk
canopies there also. In upward heating in active plage NSE retardation in
cool-after-hot cycling may affect the He I lines yet more strongly than H
$\alpha$ because the He I top is at doubled excitation energy. The
correspondence between He I D3 absorption and He II 304 Å brightness active
plage in the IBIS – SDO blinker (endnote 8) may be due to large retarded
population of the He I top (cf. Golding et al. 2017). [[GoBack Button: Back]]
[Main call].

The second display I link to is an appetizer for things hopefully to come:
comparable active regions in H $\alpha$ and Ly$\hskip 0.86108pt\alpha$ in Fig.
2 (pdf 6) of Rutten (2017b) [89 Ly$\hskip 0.86108pt\alpha$
dream]8989endnotetext: Ly$\hskip 0.86108pt\alpha$ dream
Let me dream further re imaging spectroscopy in Ly$\hskip 0.86108pt\alpha$
(footnote 62; paradise = $5\times 5$ arcmin field, 0.1 arcsec resolution, 20
wavelengths all at 1 s cadence)127127127Yet rosier dreaming: plus imaging
spectroscopy in He I 584 Å. and compare that with what we know (awake) from
SST/CRISP in H $\alpha$128128128SST/CHROMIS H $\beta$ may be yet better
because it images features sharper ($\sim\\!\\!D/\lambda$) and with less
saturation (smaller transition probability, paragraph 29 (pdf 80) of Allen
1976). Adding less saturated higher Lyman lines enabling intercomparison is
also desirable.. Co-observing with the Chinese ASO-S/LST (Ly$\hskip
0.86108pt\alpha$) and CHASE (H $\alpha$) space missions may come closest in
the near future. Ly$\hskip 0.86108pt\alpha$ and H $\alpha$ are both H I
$\alpha$ and even share the $n\\!=\\!\\!\,2$ level but differ tremendously.
Any feature visible in H $\alpha$ has much higher opacity in Ly$\hskip
0.86108pt\alpha$ that will only show its outer surface. Within hot and dense
features as internetwork shocks, dynamic fibrils, spicules-II, EBs and EB-like
lower-atmosphere reconnection sites Ly$\hskip 0.86108pt\alpha$ has fast
collision-up rates [SSX (pdf 166)] while remaining boxed-in even though being
champion resonance scatterer (many random steps but of small length [SSX (pdf
90)]). The high radiative 1–2 up and down rates then nearly cancel to net
radiative rate zero [SSX (pdf 165)], the $n\\!=\\!\\!2$ level gains near-
Boltzmann population, Ly$\hskip 0.86108pt\alpha$ has
$S\\!_{\nu}^{l}\\!\approx\\!\\!\,B_{\nu}(T)$ and H $\alpha$ has SB line
extinction. H $\alpha$ does not show such features in its core when they are
underneath the opaque H $\alpha$ fibril canopies spreading from network and
then samples them only in its outer wings, bright for EBs but Doppler-shifted
thermally-broadened dark for spicule-II RBEs and RREs. H $\alpha$ also skips
internetwork shocks by its opacity chasm in the upper photosphere (endnote 6,
[SSX (pdf 91)]). Ly$\hskip 0.86108pt\alpha$ does not have such gap so that
shifting my narrow dream passband from core to wing means sampling the whole
non-coronal atmosphere down to the low (dark) photosphere (first plot of [SSX
(pdf 90)]). In addition, Ly$\hskip 0.86108pt\alpha$ is the worst PRD line in
the spectrum (second plot of [SSX (pdf 90)]) but this is actually a boon
because it gives each part of the line its own source function and response,
as in the [SSF (pdf 111)] cartoon, and hence specific surface-sampling
signature in the dream-resolved emergent profile. At each wavelength the
emergent intensity does not sample the LTE source function within hot and
dense features but the outer monofrequent $\sqrt{\varepsilon_{\nu}}$
scattering decline towards the feature surface. As in endnote 3 the emergent
intensity is defined around $\tau_{\rm eff}\\!=\\!\\!\,1$, polarization and
Doppler signature around $\tau\\!=\\!\\!\,1$. Outer-wing passbands that form
in the upper photosphere should be well modelable with MURaM and Bifrost
(endnote 6). [[GoBack Button: Back]] [Main call]. These scenes both sample
hydrogen atoms but differ strikingly in appearance. The lines have discordant
formation in the ALC7 atmosphere [SSX (pdf 28)] but nevertheless should sample
common ionization features, I think with bright Ly$\hskip 0.86108pt\alpha$
grains showing dynamic heating and dark H $\alpha$ fibrils showing NLT cooling
[90 Ly$\hskip 0.86108pt\alpha$ features]9090endnotetext: Ly$\hskip
0.86108pt\alpha$ features
The Ly$\hskip 0.86108pt\alpha$ image in Fig. 2 (pdf 6) of Rutten (2017b) is
from the VAULT-II flight (Korendyke et al. 2001, Vourlidas et al. 2010,
Patsourakos et al. 2007) and is one of 17 images near disk center. A second
sequence of four limb images was assembled by Koza et al. (2009) into a limb
movie and a centerward movie. They show dense bright grains in plage and
active region of which a few change already during the 51-s movies. Koza et
al. (2009) suggested that the latter are Ly$\hskip 0.86108pt\alpha$
counterparts to H $\alpha$ dynamic fibrils (endnote 7). The dense more stable
grains seem tips of longer-lived upward heating jets that may also produce the
numerous bright grains in non-mossy hearts of network and plage in the IBIS H
$\alpha$ core-width scene (pdf 8) and contribute the upward chromosphere
heating of endnote 8. Away from the bright-grain plage areas in the Ly$\hskip
0.86108pt\alpha$ panel of Fig. 2 (pdf 6) the emission blobs are fuzzier and
appear remarkably as the short bright bushes extending towards internetwork in
the IBIS display (pdf 8), suggesting that they sample similar spicule-II-like
roundabout heating onsets. The dense NSE H $\alpha$ fibril canopies in the
lefthand panel of Fig. 2 (pdf 6) of Rutten (2017b) and in the IBIS H $\alpha$
image (pdf 6) appear dark not from sampling temperature but from H $\alpha$’s
self-made $\sqrt{\varepsilon}$ scattering decline ([SSX (pdf 91)], endnote 6)
with large NSE overopacity for cooling recombining hydrogen. These canopies
must be yet more opaque in Ly$\hskip 0.86108pt\alpha$ with
$S^{l}\\!\approx\\!\\!\,b_{2}B(T)$ superthermal internal source function
boosting but similarly sampling the monochromatic $\sqrt{\varepsilon}$
Ly$\hskip 0.86108pt\alpha$ declines towards the canopy surface, as suggested
by dark quieter areas in Fig. 2 (pdf 6) of Rutten (2017b) where they are not
out-radiated by brighter features in profile-summed brightness. The same
applies to filaments. [[GoBack Button: Back]] [Main call]. ALMA samples
hydrogen ions and may be able to measure both types of feature at fast enough
cadence to establish and correlate cause–effect delays. ALMA may also sample
hydrogen lines permitting super-sensitive chromospheric magnetometry [91
Rydberg H I candidate for ALMA]9191endnotetext: Rydberg H I candidate for ALMA
There may exist chromospheric features with sufficient Rydberg-ladder (endnote
7) NLT recombination emissivity for visibility in the H I 30-$\alpha$ line at
231.901 GHz in ALMA band 6 (Sect. 6 (pdf 13) of Rutten 2017a). Detection would
furnish sensitive chromosphere magnetometry. Carlsson and Rutten (1992)
predicted Rydberg lines up to lower level $n\\!=\\!\\!\,18$ for the MACKKL
star assuming SE. The formation spans between $\tau\\!=\\!\\!\,1$ at line
center and in the continuum at the bottom of the third panel of Fig. 1 (pdf 2)
shift upward with $n$ but remain below the MACKKL temperature minimum.
Reasonable reproduction of observed profiles is shown in Fig. 3 (pdf 3) but
for higher $n\mbox{-}\alpha$ lines the predicted line strengths exceed
corresponding observations in Fig. 6 (pdf 5) of Clark et al. (2000a).
Reasonable reproduction with standard-model SE line synthesis may be expected
for the lower $n$ lines, as for the Mg I 12-micron lines of endnote 4, because
the upper photosphere is the domain where standard models come closest to
represent a proper mean of actual reality. Both the modeling and the
higher-$n$ observations (including the limb detection of 21-$\alpha$ by Clark
et al. 2000b) suggested absence of Rydberg lines beyond
$n\\!\approx\\!\\!\,25$. However, at mm wavelengths the extinction increase
(with H I free-free taking over from H${}^{{}^{{}_{-}}}$ free-free following
Fig. 1 (pdf 4) of Rutten 2017b) causes formation at chromospheric heights
where dynamic NSE opacity boosting of the hydrogen top applies, not only for H
$\alpha$ and the ALMA continua but also for the high-$n$ Rydberg lines in the
ALMA range. Hence, dynamic long-lasting NLT overextinction of the H I top may
make 30-$\alpha$ yet higher-formed, at sufficiently low density to bring it
out of the reach of collisional ionization lowering, and also stronger. It may
be visible. It may be strongest off-limb since also the Balmer lines extend
there as far and bright as non-NLT Ca II H & K while far weaker on the disk.
Strength prediction may be done with NSE Rydberg synthesis from numerical NSE
simulations that produce spicules-II as copiously as observed. Detection of
this line enables exciting measurement of profile-resolving Zeeman splitting
because it scales $\sim\\!\lambda^{2}$ while competing Dopplerwidth scales
$\sim\\!\lambda$ [RTSA Eq. 3.66 (pdf 79)]129129129Erratum under [RTSA Eq. 3.66
(pdf 79)]: $\mbox{FWHM}\\!=\\!\\!\,2\sqrt{\ln(2)}\Delta\lambda_{\rm
D}\\!\approx\\!\\!\,1.665\Delta\lambda_{\rm D}$. Their ratio produces
significant profile signature already at 12 $\mu$m for 300 – 500 Gauss in Fig.
9 (pdf 12) of Rutten and Carlsson (1994); at hundredfold wavelength one Gauss
chromospheric sensitivity is in sight. [[GoBack Button: Back]] [Main call].

Finally, there are dark filaments in these active-region scenes suggesting
“chromospheric” neutral hydrogen gas in the corona. Filaments generally appear
long-lived but may nevertheless be made dynamically [92 filament
blobs]9292endnotetext: filament blobs
Filaments and prominences may be seen as “merely local aggregations of a
gaseous medium which entirely envelopes the sun” (Lockyer 1868) but yet they
present the weirdest and toughest features to understand in the rich solar H
$\alpha$ zoo. How can cool chromospheric gas show up endlessly in the hot
corona in these exceedingly long, slender, high-reaching features exuberantly
rich in fine structure and dynamics? Why well visible only in H and He
diagnostics? A nightmare for plane-parallel colleagues. Who ordered these? The
SST/SOUP H $\alpha$ movie of endnote 7 shows an active-region filament where
small bright bullet-like blobs run intermittently along its length (first
panel of Fig. 1 (pdf 2) in the analysis by Lin et al. 2012). Since then I have
noted many such fast disturbances running through filaments in AIA 304 Å
movies, suggesting frequent ionization-recombination cycling. I suspect that
they are not disturbances of a mean state but that they actually make and
remake filaments continuously with subsequent cooling gas in down-raining
condensations causing the extraordinary He I D3-like off-limb visibility of
prominences in H $\alpha$ and He II 304 Å and that this cannot be modeled
assuming static SE. For H $\alpha$ NLT retardation may cause large cooling-gas
overemissivity off-limb and large overextinction on the disk; let me repeat
(Sect. 7) that modest hydrogen ionization in simple internetwork shocks
already produces post-shock hydrogen-top retardation reaching 1012 H $\alpha$
overopacity (last panel of Fig 1 movie of Leenaarts et al. 2007), truly in the
“extraordinary” ballpark. For He II 304 Å off-limb brightness a likely
candidate is sheath heating around rain-down blobs in prominence threads by
the Kelvin-Helmholtz instability proposed by Hillier and Arregui (2019)
(endnote 8). For He II 304 Å darkness on disk, as for the active-region
filaments in the blinker 304 Å image (pdf 10), bound-free H I scattering is
likely (endnote 3). That filaments may reform in place as happened after the
exemplary failed Rosetta eruption of Mason et al. (2021) also suggests dynamic
refurbishment. [[GoBack Button: Back]] [Main call]. At high resolution and
fast cadence ALMA may also diagnose their formation including frequent
refurbishment and accompanying retarded-opacity visibility.

### Chromospheric and coronal heating.

An opinionated text as this should address “one of the longest standing
unsolved mysteries in all of astrophysics” 130130130J.T. Schmelz, ISSI talk
July 2021 for a covid-zoom audience., certainly a text harping on the
chromosphere since generally the blame for coronal heating is sought there –
while heating the chromosphere to observed temperatures is less glamorous but
requires more energy for its larger mass.

The literature on outer-atmosphere heating is bewilderingly varied. The
traditional emphasis for the coronal part is on loop modeling following Rosner
et al. (1978) debating AC (Alfvénic wave) versus DC (reconnection) mechanisms,
both dictated by dynamical fluxtube topography in the photosphere.
Chromospheric heating is attributed to a yet wider variety of agents. However,
regarding the chromosphere as a layer (not dynamically structured and
pervaded) and coronal loops as homogeneous structures (not dynamically
multistrand and multithermal) both seem as misleading as regarding the
photosphere a plane-parallel layer without convection, waves, magnetism.

My theme here is spectral evidence. Blinking the SDO triples (endnote 8) and
the IBIS – SDO images (endnote 8) suggests that a more important split than
“corona versus chromosphere” is “quiet versus active”. I summarize my
impressions from these representative blinkers.

The SDO triples suggest that quiet chromosphere heating is AC-type with
spicules-II fired from vorticity-rich network and producing return-flow
fibrilar H $\alpha$ and He II 304 Å canopies, the latter mapped well in the
AIA fire detector images. This is “around” heating (endnote 8) and does not
affect the corona.131131131Where the 304 and 131 Å canopies result from
turbulent sheath warming (endnote 8) they mark coronal cooling. Quiet corona
heating arises chiefly from CBPs, especially the larger ones, with DC loop-top
reconnection resulting in diffuser coronal connectivity to nearby CBP areas.
Their beam-heated feet are also well-mapped in the AIA fire detector product.
These two quiet-Sun heating agents appear remarkably unrelated, even though
both stem from MC dynamics in network and plage outside active regions.
Spicule-II quiet-chromosphere heating does not need MC bipolarity, CBP quiet-
corona heating does. Both act also but less effective in coronal holes where
coronal plumes seem an open-field CBP alternative.

The IBIS – SDO images suggest that active regions add unidentified “upward”
heating (endnote 8) feeding long-loop connectivity to distant other active
regions, likely with FIP abundance offsets (endnote 8).

##  9 Conclusion

Understanding the solar spectrum has come a long way but isn’t there yet. The
basic physics was well understood by the 1970s – no dark matter nor dark
energy in the solar atmosphere.132132132For a conflicting speculation search
ADS with abs:”axion quark” and ”solar orbiter” (not to be cited). Numerical
modeling started about then with the advent of electronic computing which was
welcome because analytic theory had reached its limit in Kourganoff (1952).
Numerical study provided great insights even though initially mostly within
the misleading straitjacket of plane-parallel atmospheres. Nowadays the major
challenge is to obtain realistic spectrum synthesis from realistic solar-
atmosphere simulations to provide guidance to interpret high-quality
observations conveying solar truth – with the proviso that “simulations are
great tools but still toys” (Martínez-Sykora). Full 3D(t) synthesis of all
spectral features for every voxel at all times of a RADYN-style 3D(t) MHD-etc
simulation is a bridge too far, so that the simulation synthesis frontier lies
in clever shortcuts (reviews by Pereira 2019 and Leenaarts 2020; NSE
simplification on p. 20 (pdf 20) of Rutten 2019).

Shortcuts as adopting 1.5D columnar synthesis ignoring non-radial matter and
radiation gradients, Holweger-style “inversion” with NLTE masking, neglect of
NLTE ultraviolet scattering and neglect of NLT memory by assuming SE may be as
misleading as believing plane-parallel models – but just as those they may be
educational steps along the way.

The tantalizing trophy is spectral understanding of the chromosphere which I
now define as the hectic domain separating dense near-LTE near-neutral
photospheric gas and tenuous near-CE ionized coronal gas, where hydrogen
intermittently ionizes and recombines cycling between neutral and plasma state
with full NLS+NLW+NLT and HD $\leftrightarrow$ MHD $\leftrightarrow$ plasma
complexity: the chromosphere ain’t stacked in layers but is dynamically
structured and unstuck in time (endnote 8).

Key property: frequent dynamic refurbishment.

Prime diagnostics: H I Balmer lines, Lyman lines, mm continua, helium lines –
all enriched by retarded opacity enhancement in cooling gas.

Lowest-hanging fruit: spicule-II heating of quiet chromosphere and CBP heating
of quiet corona. In activity: moss, the FIP effect a ripening plum, the
Wilson-Bappu relation overdue.

High-hanging fruit: 150+ years since Lockyer (1868) [SSX (pdf 12)] coined
“chromosphere” in the dawn of astrophysics ([SSF (pdf 2)] ff) we Khayyam-wise
have no explanation for his bright yellow “helios” line. Cracking this nut
likely helps to understand prominence visibilities.

Every aspect treated here must be considered: the Sun is a most beautiful
complex being. We are fortunate that our kind star offers such great inspiring
complexity to keep us on our toes.

##  10 Notes

Welcome to the yellow pages! These endnotes are take-or-leave additions adding
further explanation including linked literature, comments, opinions, and even
history (also personal).

Topic selection: personal, favoring topics that I have been involved in and
references and figures I knew best and came first to mind in this quick
writeup for the Calicut school. With preference for my own displays when these
open faster than ADS.133133133Example: compare the speeds of [SSX (pdf 60)]
and Fig. 1 (pdf 3) for the most-shown figure of solar physics. My pdfs are
AWS/S3 served and are linearized; arXiv pdfs are also linearized but publisher
pdfs on ADS may not - but consider footnote 23.

Navigation: each note ends with a link [Main call] returning to its calling
location in the main text and a link [Back] that goes there when you came from
there or back to another link that you used – working in Firefox, Ubuntu
acroread and xpdf, macOS Preview but not in Ubuntu evince, chrome, macOS
Safari (try the back button instead). Footnote numbers jump back to their
call.

Acknowledgements
I thank Ram Ajor Maurya and M.K. Ravi Varma for inviting me to teach this
subject at their Calicut school. I enjoyed my teaching sessions but suspect
that the (unseen) students came out by the same door as in they went [SSF (pdf
120)]. Earlier invitations to teach, in Bandung, Mitaka, Palo Alto, Seoul, La
Laguna, Freiburg, Newcastle and Weihai (usually a full week for this material)
led to displays SSF and SSX; I hope more will come in a post-covid future. I
also thank the organizers for requesting this writeup and endorsing
hyperlinks. F. Paletou, J. Leenaarts, T.M.D. Pereira and P.R. Young
contributed improvements. As always I relied much on ADS. ADS also taught me
pdf page opening  and keeps its robust classic linkers active since its 2019
revamp. ArXiv provides a splendid open-access ADS-linked publication platform.
Stackexchangers solved latex problems. M.J. Rutten started hosting
robrutten.nl for link speed and persistence.

Version history
March 3, 2021 – initial pre-school arXiv post.
April 29, 2021 –  second arXiv post honoring C. de Jager’s 100th birthday:
more content, faster serving.
May 7, 2021 – notes $\rightarrow$ endnotes.
June 1, 2021 – endnotes on yellow pages, third arXiv post.
July 20, 2021 – more informative reference format.
July 29, 2021 – reached deep-thought page limit 42.
August 3, 2021 – fourth arXiv post.
August 25, 2021 – fifth arXiv post.
December 15, 2021 – sixth arXiv post.

I intend to keep this text somewhat alive with updates and I welcome
corrections and suggestions for improvements. My website serves the latest
version with a summary here. ADS serves the last arXived version. ArXiv serves
all posted versions including their latex sources with my tricks (for me: hit
“Other formats”, hit “Download source”, tar xvf 2103.02369).

## References

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  * Allen (1976) Allen, C. W.,  “Astrophysical Quantities”, `1976asqu.book.....A` ADS
  * Allende Prieto and García López (1998) Allende Prieto, C. & García López, R. J.,  “A catalogue of accurate wavelengths in the optical spectrum of the Sun”, `1998A&AS..131..431A` ADS
  * Ando and Osaki (1975) Ando, H. & Osaki, Y.,  “Nonadiabatic nonradial oscillations: an application to the five-minute oscillation of the sun.”, `1975PASJ...27..581A` ADS
  * Antolin and Rouppe van der Voort (2012) Antolin, P. & Rouppe van der Voort, L.,  “Observing the Fine Structure of Loops through High-resolution Spectroscopic Observations of Coronal Rain with the CRISP Instrument at the Swedish Solar Telescope”, `2012ApJ...745..152A` ADS
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