# The catalog of Hvar Observatory solar observations

Mateja Dumbović  Luči Karbonini  Jaša Čalogović  Filip Matković  Karmen
Martinić  Akshay Kumar Remeshan  Roman Brajša  Bojan Vršnak  Hvar Observatory,
Faculty of Geodesy, University of Zagreb

###### keywords:

Active Regions, Structure; Active Regions, Magnetic Fields; Sunspots, Magnetic
Fields; Chromosphere, Quiet; Chromosphere, Active; Flares, Dynamics; Flares,
White-Light; Prominences, Active; Prominences, Dynamics; Prominences,
Quiescent

##  1 Introduction

intro

Though solar observations today are dominantly relying on satellites or large,
high-resolution ground-based observatories, small traditional solar
observatories still produce science quality data that can supplement state-of-
the art observations from modern instruments. For this reason we have decided
to compile the catalog of Hvar Observatory solar observations.

The solar telescope was installed at Hvar Observatory in 1972 (for a recent
review of the history of Hvar Observatory see Brajša, 2017) and consists of a
photospheric and chromospheric telescopes, that have parallel optical axes and
are mounted as one unit on the same German parallax mounting (Ambroz et al.,
1977; Čalogović et al., 2012). The double solar telescope thus observes
simultaneously photosphere and chromosphere, in white light (WL) and in
H$\alpha$ line, respectively. The corresponding fields of view are smaller
than the full solar disk (about 7 and 11 arcmin), i.e. typically the size of
the active region. Therefore, it is very important to specify the location of
the observed feature and its classification. Due to the atmosphere, the best
resolution is $\approx 1$ arcsec, and the instruments are capable of achieving
this resolution under favorable atmospheric conditions. However during the
usual weather conditions (turbulences, wind, thin clouds, humidity), the
telescope rarely achieves this resolution.

Given their characteristics (as described in more detail in Section
instruments) the instruments are traditionally considered as most suitable for
studying fast active processes on larger scales (Ambroz and Ruzdjak, 1982).
These include H$\alpha$ flares (e.g. Ruždjak et al., 1989), quiescent
prominences (e.g. Ruzdjak, Tandberg-Hanssen, and Staude, 1991), prominence
oscillations (e.g. Vrsnak, 1993), and eruptive prominences (e.g. Vrsnak et
al., 1988; Vrsnak, 1990; Vrsnak, Ruzdjak, and Rompolt, 1991) and Moreton waves
(Moreton, 1960; Vršnak and Cliver, 2008). We note, however, that in the whole
time span of analysed HVAR H$\alpha$ observations we did not observe a single
Moreton wave. HVAR WL observations are suitable for analysis of individual
sunspot groups. For instance, HVAR WL observations were found to be suitable
for an investigation of proper motions of sunspot groups Wöhl et al. (2003,
2004). With a FOV encompassing the size of the typical AR, HVAR WL
observations are suitable to be used for study of individual properties of
active regions, as their complexity and morphology is tightly related to the
(flaring) activity (e.g. Toriumi and Wang, 2019, and references therein).
Finally, on short time scales we might expect to observe white-light flares
(see e.g. Hudson, Wolfson, and Metcalf, 2006; Benz, 2017). We note however,
that in the whole time span of analysed HVAR WL observations we did not
observe a single WL flare. Therefore, in the catalog we focus on active
regions, flares and filaments/prominences.

Throughout its lifetime the telescope was mostly used in scheduled observing
campaigns and during observing visits. The regular chromospheric and
photospheric observations with Hvar Observatory (HVAR) double solar telescope
using modern equipment began in May 2011 after the installation, adjustment,
and testing of the new equipment (Čalogović et al., 2012). These continued
until 2019, when the regular observations were hampered and became rarer due
to technical reasons. Therefore, in this first version of the catalog we
encompass all observations performed by the double solar telescope at Hvar
Observatory in the time period May 2011 – November 2019. The data in this
period are stored in the archive111http://oh.geof.unizg.hr, which is publicly
accessible together with the observer logs.

In this study we go beyond the observers log and make basic characterisation
of observed phenomena that might be of scientific use for modern research. The
paper is organised as follows: in Section instruments we briefly describe the
instruments and observations for the catalog, in Section catalog we describe
what we focus on in the catalog and how it is built, along with providing
access to the catalog, and finally in Section case_study we demonstrate the
usefulness of the catalog in a case study combining multispacecraft and
multiwavelength observations.

##  2 Description of Hvar solar H$\alpha$ and white light telescopes

instruments

The double solar telescope at Hvar Observatory consists of two Carl Zeiss
refractors for H$\alpha$ and white light (WL) observations. The main purpose
of the Hvar solar telescope was to perform regular photospheric and
chromospheric observations with optimal angular and temporal resolution
following the observing routines of the Ondrejov Observatory (Ambroz et al.,
1977). Later, a similar imaging system with CCD cameras as at the Kanzelhöhe
Solar Observatory (KSO) was installed to complement the KSO full-disk
observations with high-resolution Hvar images of active regions (Otruba, 2005;
Otruba, Freislich, and Hanslmeier, 2008; Čalogović et al., 2012). In 2010, a
new generation of CCD cameras (Pulnix TM-4200GE 12-bit) was installed on the
telescopes, allowing a larger dynamic range (very bright or faint phenomena,
e.g. flares and prominences) as well as higher spatial resolution and time
cadence (Čalogović et al., 2012).

The photospheric (WL) telescope has a main objective with a diameter of 217 mm
and a focal length of 2450 mm. It consists of a Baader AstroSolar filter and
an iris diaphragm that both reduce the intensity of the sunlight in the
telescope. In addition, solar continuum and UV/IR blocking filters are used to
increase the contrast of sunspots and solar granulation by allowing the
spectral range around 540 nm to pass through, free of emission and absorption
lines. Such a setup has a corresponding field of view of about 11 arcmin,
which results in a theoretical resolution of 0.33 arcsec/pixel when using a
4Mpix Pulnix CCD (2048x2048 pixels). However, this resolution cannot be
achieved due to the ideal theoretical diffraction limit of 0.6 arcsec with an
objective of 217 mm and an observing wavelength band of 540 nm. Such a
resolution is also never achieved, since the atmosphere also has a
considerable influence on the resolution. Under the best seeing conditions,
the resolution can be around 1 arc second. In the system used, the images are
therefore oversampled, which helps to reduce aliasing and noise through image
processing and thus improves the image resolution.

Table 1.: H$\alpha$ and WL observation information and statistics for the Hvar Solar Telescope in the period from May 2011 to November 2019.tab˙hst˙stats | H$\alpha$ observations | WL observations
---|---|---
Field of view | 7 arcmin | 11 arcmin
Cadence | 15 sec | 60 sec
Number of observing days | 735 | 710
Number of images | 694 000 | 175 000
Total duration | 2891 h (120.49 days) | 2927 h (121.96 days)
Size in archive | 5.35 Tb | 1.40 Tb

Chrompsheric (H$\alpha$) telescope with a main objective diameter of 130 mm
consists of an energy reduction filter, an adjustable iris diaphragm, an
additional teleconverter lens, and an H$\alpha$ filter. A solar spectrum
research grade filter with a passband of 0.2 Å is used as the H$\alpha$
filter. The corresponding field of view of such a system is around 7 arcmin,
which results in 0.21 arcsec/pixel resolution with the 4Mpix CCD used. The
theoretical diffraction limit of the lens with a diameter of 130 mm and the
wavelength band of 656 nm is 1.3 arcsec. Similar to the WL telescope, the
images are therefore oversampled.

The acquisition software almost identical to the KSO (Otruba, 2005), regulates
the exposure time automatically and carries out the image selection in real
time. The software automatically selects the sharpest image from about 10
images taken by the CCD within about 1 second to avoid the blur caused by the
turbulence and to select the moments with good seeing. In normal observation
mode, a FITS image is saved every minute for photospheric observations and
every 15 seconds for chromospheric observations (see Table tab_hst_stats).

Some information and statistics about H$\alpha$ and WL observations can be
found in Table tab_hst_stats. In total, there are more than 700 days of
continuous observations with a total duration of almost 3000 hours in the
period from May 2011 to November 2019. Figure tab_hst_stats shows the
distribution of all individual observations during this period. Due to weather
conditions, the frequency of observations is denser in summer than in winter,
with some gaps in 2015 and 2019 when technical problems hampered observations.

Figure 1.:  Distribution of individual H$\alpha$ and WL observations in the
period from May 2011 to November 2019.fig˙hst-stats

The movies of the daily observations in this time period are publicly
available at Hvar Observatory web page222http://oh.geof.unizg.hr, whereas the
data (in FITS or jpeg format) is available upon request. The basic observation
information, such as which regions were observed and what were the weather
conditions, are provided in the observers log, also available at the same web
page.

##  3 Catalog of HVAR WL and H$\alpha$ observations

catalog

###  3.1 White light observations of the solar photosphere

WL

Figure 2.: Selected examples from the HVAR WL observations to describe the
general form of the McIntosh classification. The left panels show 7 classes of
the modified Zurich classification system: Class A – unipolar, no penumbrae;
Class B – bipolar, no penumbrae; Class C – bipolar, penumbra on one end of the
group; Class D – bipolar, penumbra on spots at both ends of the group, length
$<10^{\circ}$; Class E – bipolar, penumbra on spots at both ends of the group,
$10^{\circ}<$length$<15^{\circ}$; Class F – bipolar, penumbra on spots at both
ends of the group, length$>15^{\circ}$; Class H – unipolar, penumbra. The
middle panels show 6 classes of the penumbra of the largest sunspot: Class x –
no penumbra; r – rudimentary penumbra; s – small, symmetric penumbra; a –
small, asymmetric penumbra; h – large, symmetric penumbra; k –large,
asymmetric penumbra. The right panels show 4 classes of the sunspot
distribution: Class x – undefined relative spottedness (for unipolar groups);
Class o – open spottedness (few and small interior spots); Class i –
intermediate spottedness (numerous interior spots, but without penumbra);
Class c – compact spottedness (numerous interior spots, some with
penumbra)fig˙class1

To build HVAR WL observations catalog we focus on the classification of the
active regions observed. One of the early categorisations used for sunspot
groups is the Zurich sunspot classification, which attempts to describe the
evolution of the sunspot group to increasingly larger structures. This
categorisation was later modified by McIntosh (1990) into a form used
nowadays. The modified Zurich sunspot classification takes into account not
only the size of the group, but also whether or not the penumbra is present
and how it is distributed and consists of 7 classes (for details see McIntosh,
1990). The modified Zurich sunspot classification is supplemented by the
classification of the properties of the penumbra of the largest sunspot and
sunspot distribution into a joint McIntosh classification (McIntosh, 1990).
The general form of this classification is Zpc, where Z is the modified Zurich
class, p is the class of the penumbra of the largest sunspot, and c describes
the sunspot distribution (for details see McIntosh, 1990)333see also the SWx
classification website: https://www.stce.be/educational/classification. There
are 60 possible Zpc combinations that make McIntosh classification made by
combinations of 7 Z-classes, 6 p-classes, and 4 c-classes (note that not all
Z, p, and c classes can be combined with one another). Different Z, p, and c
classes are shown in Figure fig_class1 on selected examples from the HVAR WL
observations.

Figure 3.: Selected examples from the HVAR WL observations (left panels)
alongside HMI observations (right panels) to describe the general form of the
Mt Wilson classification. Top-to-bottom panels show: Class $\alpha$ –
unipolar; Class $\beta$ – simple bipolar; Class $\beta\gamma$ – complex
bipolar; Class $\beta\delta$ – Class $\beta$ with $\delta$ spot; Class
$\beta\gamma\delta$ – Class $\beta\gamma$ with $\delta$ spot. $\delta$ region
marks a region of closely placed opposite polarity umbrae within a common
penumbra. fig˙class2

Next, we classify observed active regions according to the Mount Wilson
classification, which refers to the magnetic structures of active regions
(ARs). For that purpose we use the observations from the Solar Dynamics
Observatory (SDO Pesnell, Thompson, and Chamberlin, 2012) Helioseismic and
Magnetic Imager (HMI Schou et al., 2012) available through JHelioviewer
service (Müller et al., 2017). Mt Wilson classification scheme is based on
classes devised by Hale et al. (1919) describing 4 basic magnetic types and 3
more complex classes derived from simple Hale classes added by Künzel (1965).
Out of 7 different Mt Wilson classes, 5 are shown in Figure fig_class2 on
selected examples from the HVAR WL observations alongside HMI observations (we
do not find examples of classes $\gamma$ and $\gamma-\delta$ in HVAR WL
observations).

The active regions were inspected visually by the observer (L.K.) using movies
of the daily observations available at Hvar Observatory web page. Since HVAR
data lacks information on the relative distance from the solar center, we did
not determine the spatial extent of the active region using HVAR data, but
instead we compared HVAR data with full disc WL images available at
SolarMonitor444https://www.solarmonitor.org and JHelioviewer (Müller et al.,
2017), where the spatial extent was measured by the observer using the grid of
stonyhurst longitude and latitude and the polarities were examined visually
using SDO/HMI observations. Based on the visual inspection and the
determination of the spatial extent the observer determined the classes of the
active regions using criteria described by Figures fig_class1 and fig_class2.
In addition, we cross-check our observations with the full-disc photospheric
observations available at SolarMonitor555https://www.solarmonitor.org. In our
catalog we provide NOAA number of the AR based on the visual comparison of
HVAR WL and SolarMonitor observations for a given day. The SolarMonitor
provides McIntosh and Mt Wilson classifications for each observed AR which in
most cases agrees with the one we derived from HVAR WL observations, but not
always. This is likely related to our inability to compare HVAR WL
observations at an exact time with SolarMonitor daily image based on which the
AR class was derived.

In addition to classifying the observed ARs, we also provide a measure of the
quality of the observation, as determined by the observer, the quality index,
QI. QI ranges from 1 (worst) to 5 (best). Note that QI is a somewhat
subjective measure, as it encompass both the “true” image/data quality (due to
e.g. weather and atmospheric conditions) as well as the observers uncertainty
in categorising the event. Nevertheless, we find that the QI might be a useful
information for future users of the catalog, especially given that it was
consistently assigned by the same observer.

Table 2.: The statistics of the AR classes in the HVAR WL catalog according to McIntosh and Mt Wilson classificationtab˙ARs McIntosh class |  |  |  |  |  |  |
---|---|---|---|---|---|---|---
| A | B | C | D | E | F | H
| 59 | 89 | 234 | 307 | 169 | 56 | 186
| x | r | s | a | h | k |
| 148 | 95 | 301 | 306 | 47 | 203 |
| x | o | i | c |  |  |
| 245 | 525 | 173 | 157 |  |  |
Mt Wilson class |  |  |  |  |  |  |
| $\alpha$ | $\beta$ | $\beta-\gamma$ | $\gamma$ | $\beta-\delta$ | $\beta-\gamma-\delta$ | $\gamma-\delta$
| 244 | 375 | 280 | 0 | 26 | 175 | 0

We observe and catalog a total of 1100 ARs, most of which are of moderate
complexity: C or D modified Zurich class, with small symmetric or asymmetric
penumbra present and of open spottedness. Regarding the Mt Wilson
classification, more than 50% of all ARs are of either $\beta$ or
$\beta-\gamma$ class. The full statistics of the AR classes observed is given
in Table tab_ARs. The catalog of ARs observed in HVAR WL is publicly available
through figshare666https://doi.org/10.6084/m9.figshare.23256890.v1.

We compare HVAR WL AR observation and classification to those cataloged by the
U.S. Air Force (USAF) solar region reports from three observatories Holloman
(HOLL), Learmonth (LEAR) and San Vito dei Normanni (SVTO) available through
National Geophysical Data Center (NGDC) of the National Oceanic and
atmospheric administration (NOAA)777https://www.ngdc.noaa.gov/stp/space-
weather/solar-data/solar-features/sunspot-regions/usaf˙mwl/, available in the
time period up to July 2017. We thus consider time period from May 2011 until
July 2017. In this time period HVAR catalogs 1017 ARs, HOLL 8671, LEAR 9233,
and SVTO 8681. In Figure fig_WL_stats1 we compare relative frequencies of
different classes cataloged by 4 observatories in the aforementioned time
period. We can see that for modified Zurich classes there are no notable
differences between HVAR and other observatories. There is however a notable
difference in the Mt Wilson classification. We can see that while other
observatories mostly have ARs classified as alpha and beta, there is a
significant proportion of ARs classified as $\beta-\gamma$ and
$\beta-\gamma-\delta$ in HVAR catalog. This could be related to a possible
HVAR bias towards selecting and tracking more complex and flare-rich ARs to
observe due to small FOV.

Figure 4.: Relative frequencies of observed AR classes cataloged by HVAR,
HOLL, LEAR and SVTO observatories in the time period from May 2011 until July
2017. a-c correspond to the modified Zurich classes and d corresponds to the
Mt Wilson classification. fig˙WL˙stats1

In order to check this potential bias we analyse total observing time
dedicated to a specific AR of specific class. Note that a particular AR can be
observed on multiple occasions and the class of the AR can change during the
total observing time. Thus, for the purpose of this analysis we associate a
particular AR with a class that was appointed to it at the time of first
observation. Figure fig_WL_stats2 shows relative frequencies of the total
observing time dedicated to ARs of different AR classes. It can be seen that
there is a slight tendency that more complex ARs are tracked for longer
periods.

Figure 5.: Relative frequencies of the total observing time dedicated to ARs
of different AR classes. a-c correspond to the modified Zurich classes and d
corresponds to the Mt Wilson classification. Note that in the d-panel we
omitted classes which were not observed.fig˙WL˙stats2

###  3.2 H$\alpha$ observations of the solar chromosphere

Halpha

In building the HVAR H$\alpha$ catalog we focus on observations of
prominences, filaments, and solar flares. We focus only on conspicuous
prominences/filaments and flares and mark the duration of their observation in
the HVAR data archive as well as some basic properties. As “conspicuous” we
consider prominent features that are either well observed, large, and/or very
active and interesting/unusual. We note that this introduces a certain bias of
the observer, nevertheless our motivation was to constrain the catalog to
features which 1) we could characterise by more than sheer time of observation
and 2) might be interesting to compare with associated coronal mass ejections
and ultimately their interplanetary counterparts. Furthermore, each feature is
marked by a quality index, QI, similarly as with ARs described in Section WL.
We note that in the case of active and eruptive phenomena such as prominence
oscillations, filament eruptions, and flares additional factor influencing the
QI is the duration of observation (e.g. whether the observations include all
stages of the phenomena from the pre-activity stage to the post-activity
stage).

The prominences/filaments are first characterised according to their location
and whether we observe them as bright (prominence) or dark feature (filament).
Based on where they are located, we differ quiescent, AR, and intermediate
filaments. Quiescent filaments are most common, they are usually found well
away from ARs, extend to large heights, and can typically last for two to
three solar rotations. AR filaments are relatively long and narrow and are
usually too low to appear as prominences at the limb, whereas intermediate
filaments are found at the borders of AR. Intermediate and AR filaments are
located at mid-latitudes, while quiescent filaments may exist over all
latitudes on the Sun (Foukal, 2004; Mackay et al., 2010). For active and
intermediate filaments/prominences we identify relevant AR (marked by NOAA AR
number in the catalog) by comparison with SolarMonitor or JHelioviewer full
disc images in H$\alpha$ when possible, otherwise using SDO Atmospheric
Imaging Assembly (AIA Lemen et al., 2012) 304 Å wavelength observations
through JHelioviewer.

Figure 6.: Examples of sinistral (left) and dextral (right) filaments from the
HVAR H$\alpha$ catalog with left-bearing and right-bearing barbs depicting
left-handed and right-handed chirality, respectively. The spine and barbs are
highlighted yellow in the bottom panels to guide the eye.fig˙chirality

We next analyse the morphology of the filament, during its “quiet” phase (when
it is not active) to obtain “magnetic insight” into the magnetic structure
that holds them. Namely, depending on their orientation, filament channels can
be dextral or sinistral, corresponding to the positive and negative helicity
of the flux rope, respectively. A typical filament is characterised by the
spine, legs, and barbs (small extensions from the spine), where the chirality
is reflected in the observation of the filament barbs. If the barbs extend
from the filament spine from left-to-right, the chirality is dextral (positive
helicity, right-handed flux rope), whereas if the barbs extend right-to-left,
the chirality is sinistral (negative helicity, left-handed flux rope) (Martin,
1998; Palmerio et al., 2017; Parenti, 2014). Examples of the two types of
filaments are shown in Figure fig_chirality. Note that these features are not
equally well recognised in all filaments. Filaments form within filament
channels, usually situated along polarity inversion lines (PILs). Not all
filament channels are filled with filament plasma, though the observation of
empty filament channels is rare (Parenti, 2014). However, sometimes only
segments of a filament channel is filled with filament plasma, thus this might
hamper the observation of their properties in H$\alpha$.

Note that while some filaments never show any notable activity, some
eventually start to oscillate or erupt. These eruptions are violent phenomena
during which the prominence disappears and are often accompanied by solar
flares and coronal mass ejections (see e.g. Schmieder, Démoulin, and Aulanier,
2013, and references therein). For these active filaments we additionally aim
to characterise type of activity. We characterise the prominence/filament
eruption as full (filament fully dissapears), partial (only part of the
filament erupts), or failed (filament starts to erupt but fails). The eruption
is usually preceded by an activation phase, during which the prominence may
exhibit mass motions in its spine or footpoints (e.g. oscillations),
fragmentary brightening and fading (due to heating), EUV brightening (due to
reconnections), deformation and helical rotation, or slow lift-off (Parenti,
2014). But this may not always be easily visible/recognisable in HVAR
H$\alpha$ observations, therefore we do not characterise the activation phase.
We note that we did try to recognize oscillations of prominences/filaments,
which are observed either as oscillatory motions or winking in H$\alpha$ (e.g.
Vrsnak, 1993). Typical oscillation periods range from 32 to 110 minutes (Luna
et al., 2018), and the velocity amplitudes are typically around tens of km/s.
Thus, using daily observation videos which do not remove the atmospheric
effects (shaking of the images) or FOV changes hamper observation of filament
oscillation. While we do not exclude the possibility of observing and
analysing possible prominence/filament oscillation in HVAR H$\alpha$
observations, it is beyond the scope of this study.

Figure 7.: Left: Relative frequencies of observed filament type per location
(AR=active region, IP=intermediate, QS=quiescent) for HVAR and AIA filament
eruption catalogs; Right: Relative frequencies of observed eruption type
(F=full, P=partial, C=confined) for HVAR and AIA filament eruption
catalogs.fig˙filament˙stat

We catalog a total number of 276 filaments and prominences in 366 observations
(some are observed multiple times). A large majority are filament observations
(78.7%) and most of the filament/prominence observations are quiescent
(50.8%), whereas we find 28.7% observations of intermediate
filaments/prominences and only 20.5% from ARs. We are able to determine
chirality in 96 filament observations, with 57.3% of them estimated as right-
handed. Finally, we observe eruption of the filament/prominence in 92 cases,
out of which 28.3% are observed as full eruptions, 56.5% as partial eruptions
and 15.2% as failed (or confined) eruptions. The catalog of HVAR
filament/prominence observations is publicly available through
figshare888https://doi.org/10.6084/m9.figshare.23530416.v1.

Figure 8.: Examples of flare types in HVAR H$\alpha$ flare catalog: a) flare
with post eruptive arcades; b) flare with asymmetric ribbons; c) flare with
circular ribbon; d) symmetric two-flare ribbon.fig˙flare˙type

We compare list of HVAR H$\alpha$ eruptive prominences/filaments with AIA
Filament Eruption Catalog999https://aia.cfa.harvard.edu/filament/ (McCauley et
al., 2015), which catalogs filament eruptions using full disc solar images in
171, 193, and 304 A wavelengths. In the HVAR observation time period AIA
Filament Eruption Catalog detected 829 eruptive filaments (almost 10 times
more). We compare the number of active region, intermediate and quiescent
filaments in the two catalogs. The comparison is shown in the left panel of
Figure fig_filament_stat. We can see that there are no notable differences
between the two catalogs. We also compare the number of full, partial and
failed/confined eruptions in the two catalogs (right panel in Figure
fig_filament_stat), where we observe a slight bias in HVAR catalog towards
observation of partial eruptions. This could be related to the fact that the
observer is likely to choose to observe a filament which already showed some
activity.

Next we catalog prominent flares, where each flare is associated with an AR
(NOAA AR number) and the GOES class via comparison with the entries in the
Solar Monitor. We additionally categorise flares according to their observed
eruptive signatures, i.e. whether their H$\alpha$ signatures include loops or
ribbons. The loops are often referred to as post eruptive arcades (PEA) or
post flare loops and are typically observed off the disc, whereas the ribbons
are observed on disc. Note that the ribbons are situated at the footpoints of
both hot and cool loops, where the inner edge of the cool H$\alpha$ loops
roughly correspond to the inner edge of the ribbons, whereas the outer edge of
the ribbons is typically associated to hot X–ray loops, not observed here.
During the course of a flare, the separation between the ribbons increases,
and the loops grow larger with time (e.g. Priest and Forbes, 2002). We
additionally characterise flare according to the shape of their ribbons as
symmetric two-ribbon flares, asymmetric, or circular-shaped. The four examples
of flare types are shown in Figure fig_flare_type.

Figure 9.: Left: Relative frequencies of observed flare classes in Solar demon
flare catalog and HVAR catalog; Right: Relative frequencies of observing time
dedicated to a flare for different flare classes in HVAR
catalog.fig˙flare˙stat

The catalog contains a total of 91 prominent flares, most of which are GOES
C-class flares (52), with 33 M-class flares and only 6 X-class flares. We
additionally checked WL observations during 6 detected X-class flares
searching for WL flares, but we found no WL signatures corresponding to the
observed H$\alpha$ flares. We specifically double checked each GOES X-class
flare observed by HVAR H$\alpha$ telescope in HVAR WL observation and HMI
continuum observation. We found one WL flare observed by HMI continuum (barely
visible) which is not observed by HVAR WL. In addition, one WL flare observed
by HMI continuum was not seen in HVAR WL observation, because the start and
peak of the flare was missing in the HVAR observations. We find that majority
of flares are observed as ribbons on disc (86) with only 5 flares showing
clear signatures of post-eruptive arcades. Regarding the ribbons, most flares
are characterised as having asymmetric ribbons (71), with 8 flares appearing
as circular-shaped ribbons and only 7 clear symmetric two-ribbon flares. The
catalog is publicly available through
figshare101010https://doi.org/10.6084/m9.figshare.23533755.v1.

Figure 10.: Examples of reverse J-shaped (left-handed) and J-shaped (right-
handed) two flare ribbons are given in the left and right panels,
respectively. The ribbons are highlighted red and the reverse J-shape and the
J-shape, respectively, are highlighted yellow in the bottom panels to guide
the eye.fig˙ribbons

We compare list of HVAR H$\alpha$ flares with a catalog of flares detected in
full disc EUV images, Solar Demon111111https://www.sidc.be/solardemon/
(Kraaikamp and Verbeeck, 2015). In the HVAR observation time period Solar
demon detected 5183 flares of C-class or higher (4572 C-class flares, 567
M-class flares and 44 X-class flares). The comparison is shown in the left
panel of Figure fig_flare_stat where it can be seen that the relative
frequency of M and X-class flares is higher in HVAR catalog, whereas the
relative frequency of C-class flares is lower. In addition, we analyse how
much observing time is dedicated to different types of flares in the HVAR
catalog (see right panel in Figure fig_flare_stat. We find that X-class flares
tend to be observed longer than C and M-class flares. HVAR observations
therefore seem biased towards more frequently observing stronger flares and
observing them in longer time periods.

Finally, we additionally analyse symmetric two-flare ribbons. The
characteristics of ribbons can indicate the helicity of the corresponding
eruptive structure (i.e. flux rope). A characteristic double J-shape and
reverse J-shape of the two flare ribbons indicates positive and negative
chirality, respectively (Démoulin, Priest, and Lonie, 1996; Palmerio et al.,
2017). Another indication of the helicity of the corresponding eruptive
structure can be the displacement of the ribbons along the PIL, which is
related to the shear. Assuming the PIL lies between the two ribbons, for
positive chirality the left ribbon is displaced downwards and the right ribbon
upwards and vice versa for negative chirality (Démoulin, Priest, and Lonie,
1996; Palmerio et al., 2017). The examples of two distinct chiralities are
given in Figure fig_ribbons and all 7 symmetric two-flare ribbons with their
properties are listed in Table tab_ribbons.

Table 3.: List of symmetric two-flare ribbons from the HVAR H$\alpha$ flare catalog with their estimated chirality.tab˙ribbons date | start time | end time | NOAA AR | GOES class | chirality | QI
---|---|---|---|---|---|---
2011-08-02 | 05:46 | 07:41 | 11261 | M1.4 | R | 3
2012-05-12 | 13:20 | 14:37 | 11476 | C2.5 | R | 2
2013-06-19 | 07:29 | 08:23 | 11776 | C3.5 | L | 1
2013-10-26 | 10:55 | 14:48 | 11882 | M1.8 | L | 3
2014-07-06 | 08:08 | 08:58 | 12109 | C2.9 | L | 2
2014-07-25 | 06:58 | 07:42 | 12121 | C2.2 | L | 2
2015-07-02 | 05:58 | 07:15 | 12373 | C1.4 | R | 2

##  4 Case study

case_study

Figure 11.: Flare and active region overview in HVAR H$\alpha$ and WL
observations (upper left and middle panels, respectively), SDO/HMI magnetogram
(lower middle panel) and SDO AIA 193 A and 304 A wavelengths (right upper and
lower panels, respectively). HVAR H$\alpha$ figure is repeated in the lower
left panel with ribbons outlined red and J-shape outlined yellow, to guide the
eye. The rough estimate of the PIL orientation is highlighted with the blue
line in SDO/HMI magnetogram figure.fig˙LCS

We demonstrate the feasibility of the catalog on a case study of the flare
detected on 2 August 2011 and related Sun-to-Earth phenomena. The event is
listed in the HVAR H$\alpha$ flare catalog as a two-ribbon flare with start of
observation 2 August 2011 05:46 and end of observation 07:41 UTC. The
associated GOES X-ray class is M1.4 according to Solar Monitor and M2
according to Solar Demon. The flare is observed in AR11261, which, according
to the HVAR WL catalog, is of Fkc and $\beta\gamma\delta$ classes according to
the McIntosh and Mt Wilson classifications, respectively. At the time of the
eruption the AR is located around N15W15. In the same AR a small filament
erupts around 06:31 UTC south west of the flare ribbons, most likely triggered
by instability caused by the first eruption. However, not being very
prominent, this small filament eruption is not cataloged. According to the
GOES soft X-ray light curve the start, peak and end of the flare are 05:24,
06:26, and 09:18 UTC, respectively.

The flare is shown in Figure fig_LCS, where it can be seen that the ribbons
show a characteristic J-shape, with the left ribbon displaced downwards and
the right ribbon upwards. This indicates positive chirality, i.e. right
handedness of the corresponding flux rope. Comparing the SDO/HMI magnetogram
with HVAR H$\alpha$ image, we can see that the two ribbons are roughly aligned
with the PIL, in the NE-SW direction. This indicates tilt between 0 and -45
degrees (measured from the solar equator in the counterclokwise direction).
The ribbons are also visible in SDO/AIA 304, whereas in SDO/AIA 193 we see
bright post eruptive arcades. However, the characteristic J-shape of the
ribbons and their orientation seem slightly better visible in H$\alpha$ than
EUV.

We next try to associate the flare to a coronal mass ejection using the
temporal and spatial causality criteria from (Vršnak, Sudar, and Ruždjak,
2005) applied to the CDAW SOHO/LASCO CME
catalog121212https://cdaw.gsfc.nasa.gov/CME˙list/. According to these
criteria, a flare is most likely physically related to the CME if its start
time is within $\pm 1$h of the CME start time (derived by extrapolating CME
kinematics to solar surface) and if its position angle, measured
counterclockwise from north, is within the opening angle of the CME. The flare
position angle is $307^{\circ}$ and we find it temporally and spatially
related to the partial halo CME with first C2 appearance 2011-08-02 06:36 UTC,
which has a position angle of $288^{\circ}$ and opening angle of
$268^{\circ}$. The CME is also observed in STEREO-A and -B SECCHI/COR2
coronagraphs, thus providing us with observations from multiple vantage
points.

Figure 12.: Left: CME observation in coronagraphs from three different vantage
points and the corresponding 3D reconstruction of the CME using the GCS model
(longitude=$26^{\circ}$, latitude=$9^{\circ}$, tilt=$2^{\circ}$,
height=8$\mathrm{R_{\odot}}$, ratio=0.49, halfangle=$44^{\circ}$). Right:
Constellation of spacecraft and the direction and extent of the CME (taken
from the DBEMv3 tool, for details see main text).fig˙CME

We perform a 3D reconstruction of the CME geometry using the Graduated
Cylindrical Shell model (GCS, Thernisien, Howard, and Vourlidas, 2006). GCS
model assumes CME has a geometry of a croissant, with conical legs centered at
the Sun, connected by a toroid of varying cross-section (largest at apex,
smallest at the legs) and a pseudo-circular front. The CME geometry and
position is fully defined in GCS by six parameters: longitude of the apex,
latitude of the apex, tilt of the croissant axis (with respect to solar
equator), height of the apex, aspect ratio (i.e. opening angle of the legs)
and half angle (full opening angle of the croissant, measured from outer edges
of the legs). We perform the GCS reconstruction by manually fitting
projections of the croissant geometry to images from three vantage points
(STEREO-A/COR2, SOHO/LASCO/C2, and STEREO-B/COR2) at the time when CME is best
observed in all three spacecraft. Note that there is a slight difference in
the timing of the CME measurements between three spacecraft, because STEREO
and SOHO do not have identical time resolution. The difference of 3 minutes
however, does not significantly influence the results, as the CME is not very
fast ($v=712\,\mathrm{kms}^{-1}$, according to the SOHO/LASCO catalog). The
CME longitude and latitude (see Figure fig_CME) correspond well with the
position of the AR and the CME tilt corresponds well with the orientation of
the PIL (both low inclined).

Figure 13.: Left: in situ measurements of the ICME that arrived at Earth 4
August 2011. The panels show (top to bottom) total magnetic field and magnetic
field fluctuations (root-mean-square, RMS); GSE magnetic field components x
(red), y (blue) and z (green); plasma density, temperature and expected
temperature calculated according to Lopez (1987); and plasma flow speed and
plasma beta. The dashed line marks the shock, sheath is shaded green, whereas
the magnetic ejecta is shaded red. Right: Hodograms depicting the behaviour of
the magnetic field within borders of the magnetic ejecta in different planes.
The red arrow marks the direction of rotation.fig˙insitu

Next, we search for possible interplanetary counterpart of the CME (ICME). For
that purpose, we run the Drag-based ensemble model tool (DBEMv3 tool,
Čalogović et al., 2021) from the ESA Heliospheric Weather Expert Service
Centre131313https://swe.ssa.esa.int/heliospheric-weather. This is an ensemble
version of the drag-based model (DBM Vršnak et al., 2013), which takes into
account input uncertainties and accordingly provides uncertainty intervals for
the output. As input we use parameters obtained from the GCS reconstruction,
supplemented with initial CME kinematics from SOHO/LASCO catalog (extrapolated
to $20\,_{\odot}$, the inner boundary of the model). The DBEMv3 input used is:
start time = 11:30 $\pm$ 30 min; drag parameter = 0.2 $\pm$ 0.1
$10^{-7}\mathrm{km}^{-1}$; solar wind speed = 350 $\pm 50\,\mathrm{kms^{-1}}$
(estimated based on the observation of in situ solar wind speed near Earth
around the time of the eruption); solar wind speed = 712 $\pm
50\,\mathrm{kms^{-1}}$; CME angular half-width = 75.8 $\pm 10^{\circ}$
(derived from GCS reconstruction parameters by the tool) and longitude = 26
$\pm 10^{\circ}$. All input uncertainties were selected as suggested by the
DBEMv3 tool (default uncertainty values). DBEM forecasts the arrival of the
ICME at Earth between 5 August 2011 03:15 and 13:50 UTC, with the arrival
speed between 435 and 525 $\mathrm{kms^{-1}}$. We do indeed find an ICME
within the predicted time period. We check Richardson and Cane (2024) ICME
catalog and it has an entry of an ICME where the shock arrival is 4 August at
21:53 UTC and the start of the magnetic ejecta is 5 August 2011 05:00 UTC.
Moreover, they associate this ICME to the same CME. The average ICME speed is
430 $\mathrm{kms^{-1}}$, which is only slightly lower than predicted by
DBEMv3.

Finally, we analyse in situ measurements of the ICME, shown in Figure
fig_insitu. We use the 1 minute plasma and magnetic field data in the
Geocentric solar ecliptic (GSE) system provided by the OMNIWeb database (King
and Papitashvili, 2005). Shock arrival is observed 4 August 2011 around 22:00
UTC (DOY 216.9), followed by the highly turbulent compressed and heated plasma
of the ICME sheath region. After the sheath, the magnetic ejecta starts 5
August 2011 around 03:00 UTC (DOY 217.15), which is observed as the rotating
magnetic field and low plasma temperature, though other parameters do not show
typical characteristics (for overview on ICME sheath and magnetic ejecta
characteristics see e.g. Kilpua, Koskinen, and Pulkkinen, 2017). We can see in
the in situ measurements and more so in hodograms in Figure fig_insitu that
throughout the magnetic ejecta all three components show rotation. Based on
the behaviour of the rotation, it is possible to deduce the orientation and
chirality of the flux rope (see e.g. Nieves-Chinchilla et al., 2019).

Figure 14.: A sketch visualising the flux rope passing at some oblique angle
$\Phi$ in xy plane and $\Theta$ in yz plane in GSE coordinates. The blue
dotted circle with an arrow marks the direction of the poloidal field, whereas
the long blue dotted arrow marks the direction of the axial field. Red and
green arrows at the leading/trailing edge of the flux rope mark components of
the poloidal and axial field, respectively (in the $B_{z}$ and $B_{y}$
direction).fig˙sketch

The $B_{x}$ shows rotation from positive to negative values, indicating that
the flux rope axis is not vertical to the radial heliocentric direction.
$B_{z}$ and $B_{y}$ both show partial rotation, from negative values to
$\approx 0$. This indicates that the flux rope axis is not aligned neither
with y- or z-axis and that at the leading edge of the flux rope the poloidal
magnetic field is oriented south-west. Consequently, we would expect the
poloidal field at the back of the flux rope to be oriented north-south. As
$B_{z}$ and $B_{y}$ do not undergo full rotation, this could indicate that the
axial magnetic field counters the poloidal axial field in $B_{z}$ and $B_{y}$
directions at the back of the flux rope, i.e. that it points to the south-
west. This is a configuration of a right-handed flux rope of positive
chirality, in agreement with the chirality estimated based on the flare
ribbons. A sketch visualising this is shown in Figure fig_sketch.

##  5 Summary and Conclusions

summary

We compile the catalog of Hvar Observatory solar observations in the time
period corresponding to regular digitally stored chromospheric and
photospheric observations 2010-2019. We make basic characterisation of
observed phenomena and compare them to catalogs which are based on full disc
solar images.

Given its resolution and FOV, HVAR WL observations are suitable to be used for
study of photospheric morphology and activity related to ARs and WL flares. We
thus compile a catalog of observed ARs consisting of 1100 entries, where each
AR is classified according to McIntosh and Mt Wilson classifications. We
search for WL flares in HVAR WL observations for the given time period, but we
do not find any. The comaprison of HVAR AR catalog with catalogs based on full
disc solar observations reveals that HVAR observations are biased towards
towards more frequently observing more complex ARs and observing them in
longer time periods. This is most likely related to the fact that HVAR
telescopes have small FOV that does not cover the whole solar disc. Therefore,
the observer has to choose what part of the Sun to observe and is likely to
choose a place where they expect more activity.

Regarding the HVAR H$\alpha$ observations, we find that the most suitable
phenomena to be observed and analysed are prominences/filaments and flares.
There is also the possibility of observing Moreton waves, but in the given
time span, we do not observe any. We thus focus on cataloging flares and
prominences/filaments. In order for the catalog to hold more information than
simply observation time, we only catalog conspicuous phenomena, which we can
characterise. We thus compile a catalog of prominences/filaments which we
characterise according to their location, chirality (if possible) and eruptive
signatures. The catalog contains a total number of 276 filaments and
prominences in 366 observations (some are observed multiple times), where we
are able to determine chirality in 96 filament observations (26.2%) and find a
total of 92 eruptive filaments (25.1%). The comparison of the eruptive
filaments with the AIA eruptive filament catalog reveals that there is a
slight bias in HVAR catalog towards observation of partial eruptions. This
could be related to the fact that the observer is likely to choose to observe
a filament which already showed some activity.

In the flare catalog we focus on their observed eruptive signatures (loops or
ribbons) and their shape. Out of 91 cataloged flares majority are observed as
ribbons on disc (86, i.e. 94.5%) with only 5 flares (5.5%)showing clear
signatures of post-eruptive arcades. Regarding the ribbons, most flares are
characterised as having asymmetric ribbons (71), with 8 flares appearing as
circular-shaped ribbons and only 7 clear symmetric two-ribbon flares, which we
additionally analyse to determine the chirality of the associated erupting
structure. the comparison of the flare catalog with a catalog of flares
detected using full disc solar images reveals that HVAR observations seem
biased towards more frequently observing stronger flares and observing them in
longer time periods.

Finally, we demonstrate the feasibility of the catalog on a case study of the
flare detected on 2 August 2011 and related Sun-to-Earth phenomena. The
analysis of the flare characteristics observed in HVAR H$\alpha$ observations
reveals a right-handed structure of low inclination. We associate the flare to
a CME and perform 3D reconstruction, which is in agreement with the low
coronal signatures. We next associate an ICME to the CME, where we find that
the magnetic structure of the ICME shows inclination and chirality in good
agreement with the solar observation.

We note that the movies of the daily observations used for this study are
publicly available at Hvar Observatory webpage141414http://oh.geof.unizg.hr,
the data (in FITS or jpeg format) is available upon request and the catalogs
are publicly available via
figshare151515https://figshare.com/s/da38ce807968ecf92bd3161616https://figshare.com/s/24b2f98a4fbc271aab5d171717https://figshare.com/s/85b87d210b12a1d4c0f6
(NOTE: these are private figshare links, public ones with DOI will be added
once the paper is accepted and the list is published in figshare.)

#### Acknowledgements

This research has received financial support from the European Union’s Horizon
2020 research and innovation program under grant agreement No. 824135
(SOLARNET). M.D. acknowledges the support by the Croatian Science Foundation
under the project IP-2020-02-9893 (ICOHOSS). We thank the team that maintains
the CDAW SOHO/LASCO CME catalog. This CME catalog is generated and maintained
at the CDAW Data Center by NASA and The Catholic University of America in
cooperation with the Naval Research Laboratory. SOHO is a project of
international cooperation between ESA and NASA. We thank the science teams of
SDO, SOHO, STEREO, ACE and Wind for providing the data and maintaining the
spacecraft. This study has made use of the JHelioviewer software provided by
ESA. We thank Hvar Observatory technicians, Nikša Novak and Toni Visković for
performing the regular observations with the solar telescope. Our special
thanks goes to late Vlado Ruždjak, who for years coordinated solar
observations at Hvar Observatory. In addition we thank W. Pötzi from
Kanzelhöhe Observatory who is persistently aiding in maintenance of the
telescopes.

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