Short-term periodicities in sunspot areas during solar cycle 22

We have analyzed the daily record of sunspot areas during the current cycle 22 looking for the short-term periodicity of around 155 days which was present during some previous solar cycles. Two different methods have been used to compute the power spectra and the results indicate that such periodicity has been absent during the current solar cycle, which confirms the results obtained by other authors who used flares or flare-related data.However, we have found that, during some intervals of time, a periodicity close to 86 days is statistically significant. A similar periodicity was found by Landscheit (1986) in energetic X-ray flares, between 1970 and 1982 (second and first half of solar cycles 20 and 21, respectively), and by Bai (1992b) for important solar flares during solar cycle 20.     

Rapid sunspot motion during a major solar flare

A major solar flare on 15 November, 1991 produced a striking perturbation in the position and shape of the sunspot related most closely to the flare. We have studied these perturbations by use of the aspect-sensor images from the Soft X-ray Telescope on board YOHKOH, and with ground-based data from the Mees Solar Observatory. The perturbation occurred during the impulsive phase of the flare, with a total displacement on the order of 1 arc sec. The apparent velocity of approximately 2 km s^–1 exceeds that typically reported for sunspot proper motions even in flare events. We estimate that the magnetic energy involved in displacing the sunspot amounted to less than 4 × 10³⁰ ergs, comparable to the radiant energy from the perturbed region. Examination of the Mees Observatory data shows that the spot continued moving at lower speed for a half-hour after the impulsive phase. The spot perturbation appears to have been a result of the coronal restructuring and flare energy release, rather than its cause.     

Periodic behavior of solar flare index during solar cycles 20 and 21

Short-term periodicities of solar activity were studied with the flare index by using Discrete Fourier Transform for the time interval 1966–1986. Two noticeable periodicities (18.5 and 5 months) have been found. The existence of these periodicities comparing with the early findings is discussed.     

Coronal rotation during solar cycle 20

Using K-coronameter observations made by the High Altitude Observatory at Haleakala and Mauna Loa, Hawaii during 1964–1976, we determine the apparent recurrence period of white-light solar coronal features as a function of latitude, height, and time. A technique based on maximum entropy spectral analysis is used to produce rotational period estimates from daily K-coronal brightness observations at 1.125R _S and 1.5R _S from disk center and at angular intervals of 5° around the Sun's limb. Our analysis reaffirms the existence of differential rotation in the corona and describes both its average behavior and its large year-to-year variations. On the average, there is less differential rotation at the greater height. After 1966–1967 we observe a general increase in coronal rotation rate which may relate to similar behavior reported for the equatorial photospheric Doppler rate. However, the coronal rate increase is significantly greater than the photospheric. If K-coronal features reflect the rotation at depth in the Sun, the long-term rate increase and the variable differential rotation may be evidence for dynamically important exchanges of energy and momentum in the upper convection zone.     

The 73-day periodicity of the flare index during the current solar cycle 22

The flare index of the current solar cycle 22 is analysed to detect intermediate-term periodicities from Sep. 1, 1986 to Dec. 31, 1991. Power spectral analysis of the time series of solar flare index data reveals a periodicity around 73 and 53 days. We find that a periodicity of 73 days was in operation from 1988 November to the end of 1991 December. We also find that when the 73-day periodicity or the 154-day periodicity is in operation, the flare index is well correlated with the relative sunspot numbers. As a conclusion, we do not expect to see a resumption of the 154-day or 73-day periodicity, but we do expect only one of the periodicity near the integral multiples of 25^d.8 in the next solar cycles.     

The 73-day periodicity of the flare index during the current solar cycle 22

The flare index of the current solar cycle 22 is analysed to detect periodicities. Power spectral analysis of the time series of solar flare index data reveals a periodicity around 73 and 53 days. We find that a periodicity of 73 days was in operation from November 1988 to the end of December 1991. We also find that when the 73-day periodicity or the 154-day periodicity is in operation the flare index is well correlated with the relative sunspot numbers.     

Global distribution of filaments during solar cycle No. 20

By tracing the positions of filaments on the solar disk for a series of consecutive Carrington rotations, one can make a compact representation of the changes in general topology of photospheric magnetic fields during the course of a solar cycle. Examples are shown for the time period 1964–1974, which may provide some insight into the long-term relationship of the mid-latitude diagonal filaments and the high latitude polar crown.     

The hemispheric variation of the flare index during solar cycles 20–23

In this paper, the monthly counts of flare index in the northern and southern hemispheres are used to investigate the hemispheric variation of the flare index in each of solar cycles 20–23. It is found that, (1) the flare index is asymmetrically distributed in each solar cycle and its asymmetry is a real phenomenon; (2) the flare index in the northern hemisphere begins earlier than that in the southern hemisphere in each of solar cycles 20–23, and the phase shifts between the two hemispheres show an odd‐even pattern; (3) although the flare index dominating in a hemisphere does not mean that it leads in phase in this hemisphere in individual solar cycle, these two features have an intrinsic relationship. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of ionizing radiation during a solar flare

Measurements of electron concentrations in the ionosphere, between 100 and 250 km altitude, were used to compute the increase in solar ionizing radiation during two flares on 21 and 23 May 1967. Since the altitude of maximum absorption of the solar energy (approximately unit optical depth) depends on the wavelength of the radiation, it is possible to estimate separately the energy enhancement in different portions of the spectrum. An ionizing energy flux increase of nearly 5 erg cm^–2 sec^–1 was observed on 21 May, while on the 23rd, the increase was over 7 erg cm^–2 sec^–1. In both flares, most of the absolute increase occurred in the 20–205 Å region of the spectrum, although the relative increase was much larger at the shorter wavelengths.     

Evolution of coronal helmets during the ascending phase of solar cycle 20

The principal polar-crown coronal helmet structures were selected from nearly three years (May, 1965–January, 1968) of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii. Six isolated and long-lived helmet systems were found at latitudes of 45° and above. Their developments are compared with underlying chromospheric and photospheric activity and a simple phenomenological model is presented showing that a coronal system is formed over an active region. Thereafter the center of gravity of the system gradually drifts poleward with the trailing unipolar magnetic region (UMR), and it becomes a high latitude coronal helmet, arched over a polar crown filament.By comparison of these coronal helmets with observations of the outer corona (to circa 4 R ) made at solar eclipse, lunar sunset, and with balloon and rocket-borne externally occulted corona-graphs, it appears that ground-based K-coronameter measurements to a distance of 1.5–2.0 R are sufficient to detect the coronal streamers.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Temporal variation of solar flare index during solar cycles 21–24

The present investigation attempts to quantify the temporal variation of Solar Flare Index(SFI)with other activity indices during solar cycles 21-24 by using different techniques such as linear regression,correlation,cross-correlation with phase lag-lead,etc.Different Solar Activity Indices(SAI)considered in this present study are Sunspot Number(SSN),10.7 cm Solar Radio Flux(F10.7),Coronal Index(CI)and MgⅡCore-to-Wing Ratio(MgⅡ).The maximum cycle amplitude of SFI and considered SAI has a decreasing trend from solar cycle 22,and cycle 24 is the weakest solar cycle among all other cycles.The SFI with SSN,F10.7,CI and MgⅡshows hysteresis during all cycles except for solar cycle 22 where both paths for ascending and descending phases are intercepting each other,thereby representing a phase reversal.A positive hysteresis circulation exists between SFI and considered SAI during solar cycles 22 and 23,whereas a negative circulation exists in cycles 21 and 24.SFI has a high positive correlation with coefficient values of 0.92,0.94,0.84 and 0.81 for SSN,F10.7,CI and MgⅡrespectively.According to crosscorrelation analysis,SFI has a phase lag with considered SAI during an odd-number solar cycle(solar cycles21 and 23)but no phase lag/lead during an even-numbered solar cycle(solar cycles 22 and 24).However,the entire smoothed monthly average SFI data indicate an in-phase relationship with SSN,F10.7 and MgⅡ,and a one-month phase lag with CI.The presence of those above characteristics strongly confirms the outcomes of different research work with various solar indices and the highest correlation exists between SFI and SSN as well as F10.7 which establishes that SFI may be considered as one of the prime activity indices to interpret the characteristics of the Sun’s active region as well as for more accurate short-range or long-range forecasting of solar events.     

North/south asymmetric entry of solar protons during the November 18, 1968 event

Solar proton observations by the ESRO IA satellite are presented for the November 18, 1968 event. The time history of proton influx over the polar regions, showing a clear north/south asymmetry during the onset phase of the event, is presented.     

Effects of hysteresis in solar cycle variations between flare index and cosmic rays

We study the hysteresis effect between the solar flare index and cosmic ray intensity for the past 37 years from January 1, 1965 to December 31, 2001 on a daily basis. We show that smoothed time series of flare index and the daily Calgary Galactic Cosmic Ray intensity values exhibit significant solar cycle dependent differences in their relative variations during the studied period. The shapes of these differences vary from cycle to cycle. So we investigate the momentary time lags between the two time series for the odd and even cycles.     

Neutron and electromagnetic emissions during the 1990 May 24 solar flare

In this paper, we are primarily concerned with the solar neutron emission during the 1990 May 24 flare, utilizing the counting rate of the Climax neutron monitor and the time profiles of hard X-rays and γ-rays obtained with the GRANAT satellite (Pelaezet al., 1992; Talonet al., 1993; Terekhovet al., 1993). We compare the derived neutron injection function with macroscopic parameters of the flare region as obtained from theHα and microwave observations made at the Big Bear Solar Observatory and the Owens Valley Radio Observatory, respectively. Our results are summarized as follows: (1) to explain the neutron monitor counting rate and 57.5–110 MeV and 2.2 MeV γ-ray time profiles, we consider a two-component neutron injection function,Q(E, t), with the form $$Q(E,t) = N_f {\text{ exp[}} - E/E_f - t/T_f ] + N_s {\text{ exp[}} - E/E_s - t/T_s ],$$ whereN _f(s),E _f(s), andT _f(s) denote number, energy, and decay time of the fast (slow) injection component, respectively. By comparing the calculated neutron counting rate with the observations from the Climax neutron monitor we derive the best-fit parameters asT _f ≈ 20 s,E _f ≈ 310 MeV,T _s ≈ 260 s,E _s ≈ 80 MeV, andN _f (E > 100 MeV)/N _s (E > 100 MeV) ≈ 0.2. (2) From the Hα observations, we find a relatively small loop of length ≈ 2 × 10⁴ km, which may be regarded as the source for the fast-decaying component of γ-rays (57.5–110 MeV) and for the fast component of neutron emission. From microwave visibility and the microwave total power spectrum we postulate the presence of a rather big loop (≈ 2 × 10⁵ km), which we regard as being responsible for the slow-decaying component of the high-energy emission. We show how the neutron and γ-ray emission data can be explained in terms of the macroscopic parameters derived from the Hα and microwave observations. (3) The Hα observations also reveal the presence of a fast mode MHD shock (the Moreton wave) which precedes the microwave peak by 20–30 s and the peak of γ-ray intensity by 40–50 s. From this relative timing and the single-pulsed time profiles of both radiations, we can attribute the whole event as due to a prompt acceleration of both electrons and protons by the shock and subsequent deceleration of the trapped particles while they propagate inside the magnetic loops.     

Particle acceleration during the explosive phase of a solar flare

Starting with the quasi-linear equation of the distribution function of particles in a regular electric field, a combined diffusion coefficient in the momentum space conbining the effects of the regular field and a turbulent field is obtained and a combined mechanism of acceleration by the regular and turbulent fields in the neutral sheet of solar proton flares is proposed. It is shown by calculation that conditions in solar proton flares are such that the charged particles can be effectively accelerated to tens of MeV, even ~1 GeV. It is shown that the combined acceleration by a regular electric field and ion-acoustic turbulence pumps the protons and other heavy ions into ranges of energy where they can be accelerated by Langmuir turbulence. By considering the combined acceleration by Langmuir turbulence and the regular electric field, the observed spectrum of energetic protons and the power-law spectrum of energetic electrons can be reproduced.     

Coronal sources of high-speed plasma streams in the solar wind during the declining phase of solar cycle 20

A correlative study is made between inferred solar sources of high-speed solar wind streams and extended white-light coronal features. The solar wind data used in the study consists of 110 co-rotating high-speed plasma streams observed from spacecraft at 1 AU in the period February 1971-December 1974; the coronal data consists of 144 equatorward extensions of polar coronal holes and 15 equatorial coronal holes, derived fromK-coronometer maps of the white-light corona during the same period. Of 110 observed solar wind streams 88 could directly be associated with an equatorward extension of a polar-cap coronal hole and 14 could be associated with a low-latitude equatorial coronal hole. In 8 cases no visible coronal feature was identified. Of 144 identified polar-cap extensions 102 were associated with a high-speed stream observed at 1 AU; 19 coronal features were related in time to data gaps in the solar wind measurements, while 38 features did not give rise to solar wind streams observed at Earth orbit. The probability of an association depended on the heliographic co-latitude of a polar hole extension, being 50% for a polar lobe extending down to 45° co-latitude and 100% for a polar coronal hole extending to 80° co-latitude or more.Paper presented at the 11th European Regional Astronomical Meeting of the IAU on New Windows to the Univese, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Thermal and nonthermal phenomena in solar flare loops at 20 cm wavelength and in X-rays

We present X-ray images from the P78-1 satellite for a long-lasting burst at 20 cm wavelength mapped with the Very Large Array on 19 May, 1979 by Velusamy and Kundu (1981). The decimeter wave observations were originally interpreted in terms of two models, one invoking thermal electrons radiating at low harmonics of the gyrofrequency, and the other invoking mildly relativistic electrons emitting gyrosynchrotron radiation. If indeed the 20 cm source is thermal, it should also be visible in soft X-rays, while if it is nonthermal, the soft X-ray emission should be weak or spatially or temporally distinct from the 20 cm burst. We find that only one of the three 20 cm sources was approximately co-spatial with the soft X-ray source, and that it was only partially thermal. The 20 cm burst is therefore primarily decimeter type IV emission from mildly relativistic electrons of the post-flare phase. The long lifetime (? 2^h) and smooth temporal variation of the burst belie its nonthermal nature and suggest continuous acceleration as well as long term storage of energetic electrons.