G. V. Rudenko

Forecasting the Velocity of Quasi-Stationary Solar Wind and the Intensity of Geomagnetic Disturbances Produced by It

A brief review is given of contemporary approaches to solving the problem of medium-term forecast of the velocity of quasi-stationary solar wind (SW) and of the intensity of geomagnetic disturbances caused by it. At the present time, two promising models of calculating the velocity of quasi-stationary SW at the Earth’s orbit are realized. One model is the semi-empirical model of Wang-Sheeley-Arge (WSA) which allows one to calculate the dependence V(t) of SW velocity at the Earth’s orbit using measured values of the photospheric magnetic field. This model is based on calculation of the local divergence fS of magnetic field lines. The second model is semi-empirical model by Eselevich-Fainshtein-Rudenko (EFR). It is based on calculation in a potential approximation of the area of foot points on the solar surface of open magnetic tubes (sources of fast quasistationary SW). The new Bd-technology is used in these calculations, allowing one to calculate instantaneous distributions of the magnetic field above the entire visible surface of the Sun. Using predicted V(t) profiles, one can in EFR model calculate also the intensity of geomagnetic disturbances caused by quasi-stationary SW. This intensity is expressed through the Kp index. In this paper the EFR model is discussed in detail. Some examples of epignosis and real forecast of V(t) and Kp(t) are discussed. A comparison of the results of applying these two models for the SW velocity forecasting is presented.

Height stratification of solar magnetic fields in cycle 23

The radial component Br of magnetic field was calculated in the potential approximation and the synoptic maps of Br for several heights in the Solar atmosphere were constructed based on observations of the photospheric magnetic field made on the old magnetograph at the US Kitt Peak National Observatory and on the new SOLIS magnetograph at the US National Solar Observatory for cycle 23 (the years 1997–2009). Parameters of large-scale structures of magnetic field with positive and negative polarities were determined at seven heights in the Sun’s atmosphere—from the photosphere (H = Ro) to H = 2.5 Ro (Ro is the Solar radius). The processes of polar reversal for polar fields and changing of the sector structure of the field at middle latitudes were observed. Characteristic lifespans and rotations were ascertained. The general picture of variations of the large-scale solar magnetic field during cycle 23 was put forward. Two types of boundaries of large magnetic structures at various heights were identified.

Determining the spatial configurations of the coronal magnetic fields of solar active regions

The fundamental possibility of reliably removing the π ambiguity from the transverse magnetic field detected in solar vector magnetographic measurements, independent of the location of the vector magnetograms on the solar disk is demonstrated. The corrected magnetograms are then used as boundary conditions for the reconstruction of the three-dimensional magnetic field. The calculated field lines agree well with observed non-potential magnetic loops. The π ambiguity is removed using a modified Metropolis algorithm adapted to a spherical geometry. The spatial configuration of the magnetic field is calculated in a nonlinear force-free approximation using an optimization method. Tests of the new algorithm for resolving the π ambiguity are demonstrated for various model cases and comparisons with results of the NPFC method.

Active regions near the recent solar-cycle minimum: Relation between plasma heating and electrical currents

Data on small active regions on the Sun collected over three years (2007–2009) are analyzed. Under very quiescent conditions (a low X-ray background level), the shapes of the coronal loops of some active regions correspond fairly well to the shapes of magnetic-field lines calculated in a potential approximation. This is true of several active regions (e.g., the group AR 10999 in June 2008) in which no flares more powerful than B3 were observed. The radio emission of this active region detected by the RATAN-600 telescope was very weak and virtually no polarization was detected. Subflares were observed in most groups. It is demonstrated using AR 10933 (January 2007) as an example that a growth in the soft X-ray emission by up to factors of ten simultaneous with an increase in the radio flux is characteristic for such active regions. A source with the opposite polarization developed to the Northwest of the main spot in AR 10933. A series of SOHO/MDI (and also Hinode) magnetograms shows the emergence of new magnetic flux before the development of this polarized source, which continued for several hours on January 8, 2007. The current density at surfaces located at various heights is estimated based on observations of the total vector magnetic field (Hinode data) and a non-linear, force-free magnetic-field extrapolation. The height-integrated current becomes appreciably stronger at two nodes above a field neutral line, near the location of the main emerging flux. This supports the idea that the emergence of new magnetic flux is a key factor in the evolution of active regions at all stages of their existence. The development of this picture could help in elucidating the inter-relationship between current enhancements, plasma heating, and particle acceleration, in both weak active regions and strong activity complexes.

Evidence for a relationship between emerging magnetic fields, electric currents, and solar flares observed on May 10, 2012

Multi-wavelength observations and magnetic-field data for the solar flare of May 10, 2012 (04: 18 UT) are analyzed. A sign change in the line-of-sight magnetic field in the umbra of a small spot has been detected. This is at least partly associated with the emergence of a new magnetic field. A hard X-ray flare was recorded at almost the same time, and a “sunquake” was generated by the impact of the disturbance in the range of energy release on the photosphere. A sigmoid flare was recorded at the beginning of the event, but did not spread, as it usually does, along the polarity inversion (neutral) line. SDO/HMI full vectormagnetic-fieldmeasurements are used to extrapolate the magnetic field of AR 11476 into the corona, and to derive the distribution of vertical currents jz in the photosphere. The relationship between the distribution of currents in the active region and the occurrence of flares is quite complex. The expected “ideal” behavior of the current system before and after the flare (e.g., described by Sharykin and Kosovichev) is observed only in the sigmoid region. The results obtained are compared with observations of two other flares recorded in this active region on the same day, one similar to the discussed flare and the other different. The results confirm that the formation and eruption of large-scale magnetic flux ropes in sigmoid flares is associated with shear motions in the photosphere, the emergence of twisted magnetic tubes, and the subsequent development of the torus instability.

Properties of the magnetic fields of coronal holes with active regions

AbstractWe determine the structure of the magnetic fields of coronal holes (CHs) and investigate its change in connection with the emergence of active regions (ARs) in CHs. Based on our observations in the HeI 1083 nm line performed with the CrAO TST-2 telescope, we have selected CHs of two types: without (15 CHs) and with (28 CHs) ARs. Magnetograms obtained at the Kitt Peak National Solar Observatory have been used to calculate the magnetic fields of the same objects.We have calculated magnetic field characteristics by Rudenko’s method in the potential approximation at several heights in the corona, namely, the average (over the CH area) radial field component 〈Br〉 and its magnitude 〈|Br|/B〉 and the maximum and minimum (over the CH area) values of Br. The distributions of the isolines of these parameters superimposed on the CH images and the field lines of the calculated magnetic field have been constructed with resolutions of 33.4″ and 100.2″ on the solar surface. Analysis of these data has yielded the following results: The field lines originating in CHs without ARs are open or very high loops that are closed outside CHs. The latter occurs in completely or partially closed CHs.The system of open loops is stable, changes little with the birth of ARs, and is completely restored after the disappearance of ARs.ARs emerging inside CHs are bipolar or multipolar magnetic structures. They are formed by closed field lines.The field lines originating in ARs are closed either inside ARs or in the immediate neighborhood of ARs in CHs.There is virtually no connection of ARs inside CHs with external ARs or other places outside CHs.

Correlation between the parameters of coronal mass ejections and features of their propagation in the corona with the large-scale structure of the solar magnetic field

Correlation between the parameters of coronal mass ejections (CMEs) that are detected on the LASCO coronographs and are associated with eruptive prominences and the distances of CME axes from the coronal streamer belt has been analyzed. The deviations of CME trajectories from the radial direction have been investigated.

Magnetic field variations accompanying the filament eruption and the flare related to coronal mass ejections

Field variations in the region of eruptive event of June 7, 2011, associated with the filament eruption (FE), flare, and coronal mass ejection are studied based on vector measurements of the photospheric magnetic field with the SDO/HMI instrument. Variations of the module (B), the radial (Br) and transverse (Bt) components of the magnetic induction, and the inclination angle (α) of field lines to the radial direction from the center of the Sun are analyzed. It is shown that the strongest changes of the field before the event were located near the base of the southeastern leg of the eruptive filament; after the beginning of the event, they were located in the CME flare region. It is suggested that the FE is associated with two episodes of strong and rapid field variations: before the beginning of the slow filament rise and before its sudden acceleration. For the first time, variations of the inclination angles of the field lines over time in different parts of the eruptive event are studied in detail. It was found that the inclination angles of the field lines decrease in the vicinity of its channel during the slow rise of the filament, and the inclination angles of the field lines increase sharply after the beginning of the flare in the flare region in the vicinity of the neutral line.

Initial formation of an “impulsive” coronal mass ejection

An “impulsive” coronal mass ejection (CME) observed on August 24, 2014 is analyzed using ultraviolet images obtained in the SDO/AIA 193, 304, 1600, and 1700 Å channels and Hα (6562.8 Å) data obtained with the EI Teide and Big Bear telescopes. The formation of this impulsive CME was related to a magnetic tube (rope) moving with a velocity of ≈35 km/s and containing plasma that was cooler than the photospheric material. Moving in the corona, the magnetic tube collides with a quasi-stationary coronal magnetic rope, with its two bases rooted in the photosphere. This interaction results in the formation of the CME, with the surface of the coronal magnetic rope becoming the CME frontal structure. According to SDO/HMI data, no enhancements or changes in magnetic flux were detected in the vicinity of the CME bases during its formation. This may support the hypothesis that the magnetic tube starts its motion from layers in the vicinity of the temperature minimum.

A comparative analysis of the properties of the magnetic fields in leading and trailing sunspots

Pairs of leading and trailing sunspots whose umbrae are joined by magnetic-field lines have been selected based on calculations using SOLIS magnetic-field data in a potential approximation and the Bd technique of Rudenko, together with SDO data for 2010–2013. The shape of the field lines reflects to some extent the shape of the magnetic tube connecting the leading and trailing spots. The minimum angle between the field lines and the radial direction amin, the maximum magnetic field Bmax, the length of the field line from the leading spot to the apex, where the radial component of the field is zero, Ll, and the length of the field line from the apex to its eastern base Lf are determined in the umbrae of all the selected sunspots. In ∼81% of cases, amin is smaller in the leading spot than in the trailing spot. For such sunspots, there is a positive correlation between these angles in the leading and trailing spots. The dependences of amin on the areas of the umbrae in the leading and trailing spots are different. There is a weak negative correlation between amin and Bmax. In other words, on average, the field lines are closer to radial in magnetic tubes forming the umbrae of both leading and trailing spots with stronger fields at the photospheric level. In ∼60–65% of cases, the section of the field adjacent to the leading spot Ll is shorter than Lf. Similar results are obtained for large single spots.