Forbush Effects Associated with Disappeared Solar Filaments

Abstract

The Forbush effects (FEs) in cosmic rays associated with interplanetary disturbances caused by the disappearance of solar filaments (DSFs) outside active regions (ARs) are considered. In total, 481 FEs were detected for 1995–2023 using the database of Forbush Effects and Interplanetary Disturbances (FEID). The behavior of the cosmic ray density was calculated using the Global Survey Method (GSM). The distributions of the FE numbers depending on their duration and magnitude, as well as on the characteristics of the interplanetary and near-Earth medium, were obtained. It is found that the average duration of such FEs (33.4 ± 0.5 h) is almost the same as for events associated with CMEs from ARs, but the average magnitude is much smaller (0.83 ± 0.03%). It is also shown that coronal mass ejections (CMEs) caused by DSFs are often low-speed interplanetary disturbances (with an average maximum SW speed of 423.2 ± 3.5 km/s), the velocities of which are close to the speed of the background solar wind (SW). During FEs associated with CMEs after DSFs outside ARs, on average, unsettled geomagnetic activity is observed. Magnetic storms were recorded only in 19% of events. Lower values of FE magnitude and geomagnetic activity are associated with weakened magnetic fields and low speeds of such interplanetary disturbances.

1. Introduction

Disappearance of solar filaments (DSF) is one of the most complex manifestations of solar activity and it is important in terms of processes affecting space weather. The plasma characteristics of solar filaments differ significantly from those of the surrounding plasma. On average, the plasma density in solar filaments is two orders higher and the temperature is two orders lower than in the surrounding plasma. The sizes and shapes of solar filaments are extremely diverse, and their lifetimes can vary from several hours to several months [1]. Filaments can be very dynamic and unstable formations, whose movement is often associated with coronal mass ejections (CMEs) and solar flares. After a sufficiently long presence of filaments in the corona high above the photosphere and chromosphere, the filaments can rise quite rapidly and then erupt [2,3]. Such a filament can become the core of a CME.

CMEs that cause non-recurrent Forbush effects (FEs) can occur both due to eruptions in active regions (ARs) and ones that are not formed within a single AR [4,5,6,7]. CMEs erupting from ARs occur in stronger magnetic fields and are therefore usually more energetic. These CMEs propagate toward the Earth with greater velocity and cause FEs of greater magnitude than CMEs caused by DSFs outside ARs [8].

FEs are the most diverse and interesting phenomena in cosmic rays, studied by many researchers [4,9,10,11,12,13,14,15]. The most complete definition of FEs is given by Belov [4], where an FE is understood as a change in the density (isotropic part of cosmic ray intensity) and anisotropy of cosmic rays in large-scale disturbances of the solar wind (SW). Disturbances of the interplanetary medium can be sporadic or recurrent [4,9,13,16]. Sporadic disturbances are associated with CMEs, and recurrent ones with high-speed streams (HSSs) from coronal holes (CHs) and their corotating interaction regions (CIRs). Sporadic disturbances of the interplanetary medium also include disturbances associated with CMEs after DSFs. It should be noted that many interplanetary disturbances have mixed solar sources, which include both CMEs and an HSS with a CIR [13,17]. Quite often, disturbances interact with each other in interplanetary space [18], causing a complication of events registered near the Earth. An interesting study was conducted in the work by Gopalswamy et al. [19], where there is a comparison of the influence on the heliosphere of CMEs from active regions and after DSFs beyond active regions (non-sunspot). The authors found that almost all energetic CMEs are associated with eruptions in active (sunspot) regions, and the frequency of occurrence of these CMEs is strongly related to the level of solar activity determined by the number of sunspots, while CMEs after DSFs beyond active regions are much less energetic, and their number does not depend on the level of solar activity. In [20], a statistical comparison of the parameters of the temporal development of FEs associated with CMEs from active regions accompanied by solar flares and with CMEs associated with DSFs was investigated. The comparison showed that FEs of these two types are characterized by the same cosmic ray decrease phase at the maximum of solar cycle (SC) 23, and the difference in the temporal profiles of FEs of the two types is more noticeable at the maximum of SC 23 than at the maximum of SC 24 and the minimum between SC 23 and 24.

The aim of this work is to investigate the distributions of the number of FEs associated with DSFs, depending on their duration and magnitude, as well as the characteristics of the interplanetary and near-Earth medium.

2. Data and Method

Information on the parameters of the SW and interplanetary magnetic field (IMF) is taken from the OMNI database (https://omniweb.gsfc.nasa.gov/). Information on geomagnetic activity (Kp and Ap indices) is taken from ftp://ftp.gfz-potsdam.de/pub/home/obs/kp-ap/wdc, (accessed on 28 April 2025) [21] and http://wdc.kugi.kyoto-u.ac.jp/index.html, (accessed on 28 April 2025) and the Dst-index from http://wdc.kugi.kyoto-u.ac.jp (accessed on 28 April 2025). To study the change in the cosmic ray intensity together with data on the state of the interplanetary and near-Earth environment, the Solar and Geomagnetic Activity database was used, maintained at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of Russian Academy of Sciences (IZMIRAN). For interplanetary disturbances analysis one more IZMIRAN database was used—FEID (Forbush Effects and Interplanetary Disturbances) https://tools.izmiran.ru/feid (accessed on 28 April 2025). Cosmic ray variation parameters in the database are computed using the Global Survey Method (GSM) [22] for 10 GV rigidity particles with the world neutron monitor network data. This cutoff rigidity value is the closest to the effective rigidity of particles recorded by neutron monitors. The GSM is a version of harmonic spherical analysis based on data from all available ground detectors www.nmdb.eu (accessed on 28 April 2025) allowing 1 h data on cosmic ray flux variations to be obtained, observed by the Earth as a single multi-directional detector. The GSM allows parameters of cosmic rays beyond the Earth’s magnetosphere to be obtained, i.e., practically in interplanetary space, which makes it possible to compare them with the parameters of the interplanetary medium. The FEID database includes almost all large-scale SW disturbances for the last several solar activity cycles (since 1957). The database also contains data on solar sources of FEs, which are based on a comprehensive analysis of solar and interplanetary data [23,24]. Firstly different SW and IMF parameters are analyzed in order to refer the FE to recurrent or sporadic ones. Then some remote and in situ observed properties of the interplanetary disturbances (e.g., SW speed, temperature, and IMF polarity) are taken into consideration to estimate the possible dates of the corresponding solar event. At the second stage, the analysis of solar data is carried out on the dates that were defined from the SW parameters. CHs, solar flares, and eruptions are clearly visible on the shots and films of the Sun (e.g., SDO data); CMEs are usually clearly visible in the coronagraph data. At the third stage, possible solar sources are compared to other available databases and catalogs of CHs and CMEs. As a result, some FEs can be associated with the corresponding solar sources and assigned to one of the following types of events: (i) FEs caused by CIRs and HSSs from CHs; (ii) FEs corresponding to flare-associated ejections from ARs; (iii) FEs corresponding to filament eruptions from the areas not formed within a single AR; and (iv) mixed sources. The described analysis of solar sources is carried out using the following open resources: https://soho.nascom.nasa.gov/, https://cdaw.gsfc.nasa.gov/CME_list/, https://ssa.esac.esa.int/ssa/#/pages/home, https://sdo.gsfc.nasa.gov/assets/img/dailymov/, https://solen.info/solar/index.html, https://solen.info/solar/coronal_holes.html, https://www.sidc.be/SILSO/datafiles, and http://www.srl.caltech.edu/ACE/ASC/DATA/level3/icmetable2.htm (accessed on 28 April 2025).

Figure 1 demonstrates the differences between the interplanetary medium, cosmic rays, and geomagnetic activity parameters between two typical events associated with CMEs from (left) and outside (right) an AR. The first event is associated with an M8 X-ray flare (16:13 UT, N12E11) and halo CME (17:12 UT, initial linear speed 1689 km/s) on 13 May 2005 (https://izw1.caltech.edu/ACE/ASC/DATA/level3/icmetable2.htm, https://cdaw.gsfc.nasa.gov/movie/make_javamovie.php?date=20050513&img1=soh_e195&img2=lasc2rdf) (accessed on 28 April 2025). The second event is associated with the eruption of a large solar filament in the north part of a solar disk on 12 December 2012 (CME with initial linear speed of 203 km/s at 08:54 UT (see https://izw1.caltech.edu/ACE/ASC/DATA/level3/icmetable2.htm (accessed on 28 April 2025) and https://cdaw.gsfc.nasa.gov/movie/make_javamovie.php?date=20081212&img1=soh_e195&img2=lasc2rdf) (accessed on 28 April 2025). One can see that SW speed, IMF strength, magnitude of FE, and level of geomagnetic activity are higher for AR-associated events.

For the current research FEs that are reliably associated with filament eruptions from the areas outside ARs (DSFs) from the FEID database were selected. Various characteristics of the interplanetary and near-Earth medium for the period 1995–2023, as well as data on solar activity, were used. This period was chosen for this study since observations are available for almost the entire period from the Solar and Heliospheric Observatory (SOHO, https://soho.nascom.nasa.gov/) and Solar Dynamics Observatory (SDO, https://sdo.gsfc.nasa.gov/) spacecraft. The onset of an FE is determined by the sudden onset of a geomagnetic storm (Sudden Storm Commencement—SSC), including events with a sudden impulse (sudden impulse—SI) defined by data from individual ground magnetometer stations or spacecraft. If these indicators are not available, abrupt changes in the main parameters of the interplanetary medium (SW velocity, interplanetary magnetic field strength, and cosmic ray intensity) are considered to be the onset of an FE.

For the study, FEs caused by disturbances of the interplanetary medium from CMEs after DSFs were selected, and these events should not contain a contribution from the influence of high-speed SW streams from coronal holes or CMEs from active regions associated with solar flares. Also, in order to avoid “superposition” of effects in different parameters due to possible interaction [13,16], the condition was met that the previous interplanetary disturbance was recorded more than a day before the studied one. For the period 1995–2023, a catalog of 481 such events was constructed (available at https://ionos.kz/files/FE-DSF.pdf (accessed on 28 April 2025)). The criteria for selecting events, in addition to the solar source described above, were the requirements that the event duration should be more than 12 h and the availability of experimental data during the event is more than 90%. Note that events with a shorter duration are rare (there are only 8) and influenced by other SW disturbances, so their analysis may not have reliable results.

3. Results and Discussion

For FEs caused by CMEs after DSFs, distributions of the number of events by duration, magnitude, and characteristics of the near-Earth and interplanetary medium were constructed. The duration of an FE is also calculated for each event in the FEID and it is chosen as the moment when 1) all the extreme values are registered and 2) the main characteristics of SW, IMF, and CR return to pre-event (background) levels or 3) simply a new event begins. Figure 2 shows the distribution of the number of events by their duration and magnitude. Note that there are two more events with magnitudes of 6 and 8.2% in Figure 2 (right panel). The magnitudes of these two events are larger because these two filaments had unexpectedly large magnetic fields and high velocities. Maximum IMF strength was 24.5 and 32 nT, and maximum SW speed was 610 and 599 km/s.

It is evident that the majority of FEs (91%) associated with CMEs after DSFs have a duration of 15 to 50 h. The average duration of events is 33.4 ± 0.5 h, and the median value is 34 h. The longest FE with the onset at 18 UT on 4 July 2013 lasted 75 h. Note that the average duration of events associated with CMEs from ARs is almost the same—34.4 ± 0.7 h—so we can conclude the place of eruption (outside AR or within it) does not matter a lot for the duration of FEs.

The magnitude of FEs from our sample varies between 0.2% and 8.2%. The average magnitude is 0.83 ± 0.03%, with a median of 0.61%. Despite the fact that the FEs we are considering are associated with CMEs, on average these FEs have a small magnitude of <1%. FEs with magnitude > 2% were recorded only in 28 events, which is 5.8% of the total number of events. This is apparently due to the fact that CMEs after DSFs outside ARs are much less energetic than CMEs associated with eruptions in active (sunspot) regions. This is consistent with the results of [20,23], who showed that more efficient CR modulation occurs in FEs associated with AR CMEs. In these studies the authors gave much larger magnitudes for AR CMEs—2.94 ± 0.18%.

The distributions of the number of FE events depending on the characteristics of the interplanetary medium were also considered. Figure 3 shows the distributions of the number of FEs depending on the maximum SW velocity (Vmax) during the events and the maximum strength of the interplanetary magnetic field (Bmax).

In total, 73% of FEs occur with Vmax changes within the range from 325 km/s to 475 km/s; i.e., low-speed interplanetary disturbances with velocities close to the background SW are common among CMEs after DSFs. Average Vmax = 423.2 ± 3.5 km/s, and the median value is 408.0 km/s. Only 78 FEs (16% of all events) occurred at Vmax ≥ 500 km/s.

The majority (90%) of the studied events have Bmax < 15 nT. Average Bmax = 9.98 ± 0.19 nT, and median value is 9.0 nT. This is an expected result because magnetic field values out of ARs are much less. For comparison, IMF strength in interplanetary disturbances caused by CMEs from ARs can reach 60 nT [23,24]. In the considered sample the maximum registered value of Bmax is 33.6 nT, which was observed during a significant FE on 11 August 2000 with a magnitude of 2.77% with Vmax = 672 km/s.

Most often, FEs are registered along with geomagnetic field disturbances. Figure 4 shows the distribution of the number of FEs depending on the maxima of Kp and Ap geomagnetic indices.

During FEs associated with CMEs after DSFs, there is, on average, an unsettled geomagnetic activity with Kpmax = 3.3 ± 0.1 and Apmax = 24.4 ± 1.2. Minor and moderate magnetic storms were observed in 82 events, strong magnetic storms in 7 events, and a severe magnetic storm (Apmax = 179) in 3 events. Thus, 19% of FEs from the studied sample were observed against the background of magnetic storms. This result is quite expected for CMEs after DSFs outside ARs, which are less energetic than CMEs from ARs.

4. Conclusions

A catalog of FEs associated with interplanetary disturbances caused by DSFs outside ARs has been created for the period from 1995 to 2023 https://ionos.kz/files/FE-DSF.pdf (accessed on 28 April 2025), indicating the main characteristics of cosmic rays, the interplanetary medium, and geomagnetic activity.

It is shown that the majority of FEs (77% of 481 events) associated with CMEs after DSFs have a duration of 15 to 40 h. The average duration of such events is 33.4 ± 0.5 h, and the median value is 34 h. The longest FE lasted for 75 h. Comparing these results with FEs caused by CMEs from ARs, we can conclude that the place of eruption (outside AR or within it) does not matter a lot for the duration of FEs.

The magnitude of FEs varies between 0.2% and 8.2%. The average magnitude is 0.83 ± 0.03%. On average, the studied FEs have a small magnitude of <1%. FEs with a magnitude of >2% were recorded only in 28 events, which is 5.8% of the total number. Such magnitudes are 2–3 times smaller than the ones for FEs associated with CMEs from ARs. This is apparently due to the fact that DSFs outside ARs have lower magnetic fields that first of all determine the FE magnitude, as most of the studied events (90%) have a value of Bmax < 15 nT, while CMEs from ARs can have IMF strength up to 60 nT.

In total, 73% of FEs under consideration occur with Vmax changes in the range from 325 km/s to 475 km/s; i.e., among CMEs after DSFs there are often low-speed interplanetary disturbances (average Vmax is 423.2 ± 3.5 km/s), the velocities of which are close to the background SW.

During FEs associated with CMEs after DSFs, on average, an unsettled geomagnetic activity is observed with Kpmax = 3.3 ± 0.1 and Apmax = 24.4 ± 1.2. Magnetic storms were registered only in 19% of events. Again such largely undisturbed geomagnetic activity can be explained by the combination of low SW speeds and IMF strengths in these interplanetary disturbances.