B. Kirov

Long-term variations in the correlation between NAO and solar activity: The importance of north–south solar activity asymmetry for atmospheric circulation

Abstract General atmospheric circulation is the system of atmospheric motions over the Earth on the scale of the whole globe. Two main types of circulation have been identified: zonal – characterized by low amplitude waves in the troposphere moving quickly from west to east, and meridional with stationary high amplitude waves when the meridional transfer is intensified. The prevailing type of circulation is related to global climate. Based on many years of observations, certain “circulation epochs” have been defined when the same type of circulation prevails for years or decades. Here we study the relation between long-term changes in solar activity and prevailing type of atmospheric circulation, using NAO index reconstructed for the last four centuries as a proxy for large-scale atmospheric circulation. We find that when the southern solar hemisphere is more active, increasing solar activity in the secular solar cycle results in increasing zonality of the circulation, while when the northern solar hemisphere is more active, increasing solar activity increases meridional circulation. In an attempt to explain the observations, we compare the short-term reaction of NAO and NAM indices to different solar drivers: powerful solar flares, high speed solar wind streams, and magnetic clouds.

What causes geomagnetic activity during sunspot minimum

It is well known that the main drivers of geomagnetic disturbances are coronal mass ejections whose number and intensity are maximum in sunspot maximum, and high speed solar wind streams from low latitude solar coronal holes which maximize during sunspot declining phase. But even during sunspot minimum periods when there are no coronal mass ejections and no low latitude solar coronal holes, there is some “floor” below which geomagnetic activity never falls. Moreover, this floor changes from cycle to cycle. Here we analyze the factors determining geomagnetic activity during sunspot minimum. It is generally accepted that the main factor is the thickness of the heliospheric current sheet on which the portion of time depends which the Earth spends in the slow and dense heliospheric current sheet compared to the portion of time it spends in the fast solar wind from superradially expanding polar coronal holes. We find, however, that though the time with fast solar wind has been increasing in the last four sunspot minima, the geomagnetic activity in minima has been decreasing. The reason is that the parameters of the fast solar wind from solar coronal holes change from minimum to minimum, and the most important parameter for the fast solar wind’s geoeffectivity—its dynamic pressure—has been decreasing since cycle 21. Additionally, we find that the parameters of the slow solar wind from the heliospheric current sheet which is an important driver of geomagnetic activity in sunspot minimum also change from cycle to cycle, and its magnetic field, velocity and dynamic pressure have been decreasing during the last four minima.

On the relation between solar activity and seismicity

Much attention is recently paid to the role of extraterrestrial factors in terrestrial seismicity, and to the possibility to assess the seismic risk. Seven centuries of records of ancient earthquakes in the Mediterranean region show that the century-scale variations in the number of strong earthquakes closely follow the secular cycle of solar activity. Two well expressed maxima in the global yearly number of earthquakes are seen in the 11-year sunspot cycle - one coinciding with sunspot maximum, and the other on the descending phase of solar activity. A day to day study of the number of earthquakes worldwide reveals that the arrival to the Earth of high speed solar streams is related to significantly greater probability of earthquake occurrence. The possible mechanism includes deposition of solar wind energy into the polar ionosphere where it drives ionospheric convection and auroral electrojets, generating in turn atmospheric gravity waves that interact with neutral winds and deposit their momentum in the neutral atmosphere, increasing the transfer of air masses and disturbing of the pressure balance on tectonic plates. The main sources of high speed solar streams are the solar coronal mass ejections (CMEs) which have a maximum in the sunspot maximum, and the coronal holes with a maximum on the descending phase of solar activity. Both coronal holes and CMEs are monitored by satellite-borne and ground-based instruments, which makes it possible to predict periods of enhanced seismic risk. The geoeffectiveness of solar wind from a coronal hole only depends on the position of the hole relative to the Earth, and for the CMEs an additional factor is their speed. It has been recently found that a useful tool in identifying the population of geoeffective CMEs is the detection of long-wavelength (decameter-hectometer) type II solar radio bursts, as the CMEs associated with them are much faster and wider than average.

An instrument for total ion drift velocity measurements aboard the ‘Intercosmos-Bulgaria-1300’ satellite

Abstract The ‘INTERCOSMOS-BULGARIA-1300’ satellite was launched on Aug. 7/81, to investigate ionospheric plasma dynamics. It had a perigee of 825 km, an apogee of 906 km and orbit inclination of 81.2°. The satellite was three axis stabilized within ±1° on each axis. The ion driftmeter, ID-1, aboard this satellite was intended to measure ion density irregularities, the ion drift velocity and photoelectron fluxes. The purpose of this paper is to present a brief description of the ID-1 instrumentation and to show the first results obtained from the flight instrument.

Long-term variations of geomagnetic activity and their solar sources

Geomagnetic activity in each phase of the solar cycle consists of 3 parts: (1) a floor below which the geomagnetic activity cannot fall even in the absence of sunspots, related to moderate graduate commencement storms; (2) sunspot-related activity due to sudden commencement storms caused by coronal mass ejections; (3) graduate commencement storms due to high speed solar wind from solar coronal holes. We find that the changes in the floor depend on the global magnetic moment of the Sun, and on the other side, from the height of the floor we can judge about the amplitude of the sunspot cycle.Geomagnetic activity in each phase of the solar cycle consists of 3 parts: (1) a “floor” below which the geomagnetic activity cannot fall even in the absence of sunspots, related to moderate graduate commencement storms; (2) sunspot-related activity due to sudden commencement storms caused by coronal mass ejections; (3) graduate commencement storms due to high speed solar wind from solar coronal holes. We find that the changes in the “floor” depend on the global magnetic moment of the Sun, and on the other side, from the height of the “floor” we can judge about the amplitude of the sunspot cycle.

Helicity of Magnetic Clouds and Solar Cycle Variations of their Geoeffectiveness

Coronal mass ejections (CMEs) are sources of the strongest geomagnetic disturbances. From sunspot minimum to sunspot maximum, the intensity of storms associated with CMEs increases but the degree of association decreases. We divide the CMEs in the last solar cycle (1996–2002) into magnetic clouds (MCs)and CMEs which are not magnetic clouds. MCs are much more geoeffective than non-MC CMEs, and the portion of CMEs which are MCs is maximum in sunspot minimum and minimum at sunspot maximum, corresponding to the net helicity transferred from the solar interior into the corona. The smaller portion of the more geoeffective MCs is the explanation of the smaller degree of association of CMEs with geomagnetic disturbances in sunspot maximum. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Long-term variations of solar magnetic fields derived from geomagnetic data

There are limited homogeneous instrumental observations of the sunspot magnetic fields, but the Earth is a sort of a probe reacting to interplanetary disturbances which are manifestation of the solar magnetic fields. We find correlations between some parameters of geomagnetic activity (the geomagnetic activity “floor”—the minimum value under which the geomagnetic activity cannot fall in a sunspot cycle, and the rate of increase of the geomagnetic activity with increasing sunspot number), and sunspot magnetic fields (the sunspot magnetic field in the cycle minimum, and the rate of increase of the sunspot magnetic field from cycle minimum to cycle maximum). Based on these correlations we are able to reconstruct the sunspot magnetic fields in sunspot minima and maxima since sunspot cycle 9 (mid 19th century).Sunspots are dark spots on the solar surface associated with strong magnetic fields. The number, area, and brightness of sunspots are supposed to reflect the intensity of the solar magnetic fields and are often used as proxies for their long-term variations. However, the correlations between the sunspot parameters and solar magnetic fields are not constant, and the causes and the time profiles of the variations in these correlations are not quite clear. Therefore, the sunspot data alone cannot be used as proxy for deriving the variations of the sunspot magnetic fields for periods when no instrumental measurements are available. But the Earth is a sort of a probe reacting to interplanetary disturbances which are manifestation of the solar magnetic fields, so records of the geomagnetic activity can be used as diagnostic tools for reconstructing past solar magnetic fields evolution. In the present study we combine sunspot and geomagnetic data to estimate the long-term variations of sunspot magnetic fields.

Langmuir probe measurements aboard the International Space Station

In the current work we describe the Langmuir Probe (LP) and its operation on board the International Space Station. This instrument is a part of the scientific complex “Ostonovka”. The main goal of the complex is to establish, on one hand how such big body as the International Space Station affects the ambient plasma and on the other how Space Weather factors influence the Station. The LP was designed and developed at BAS–SRTI. With this instrument we measure the thermal plasma parameters–electron temperature Te, electron and ion concentration, respectively Ne and Ni, and also the potential at the Station’s surface. The instrument is positioned at around 1.5 meters from the surface of the Station, at the Russian module “Zvezda”, located at the farthermost point of the Space Station, considering the velocity vector. The Multi- Purpose Laboratory (MLM) module is providing additional shielding for our instrument, from the oncoming plasma flow (with respect to the velocity vector). Measurements show that in this area, the plasma concentration is two orders of magnitude lower, in comparison with the unperturbed areas. The surface potential fluctuates between–3 and–25 volts with respect to the ambient plasma. Fast upsurges in the surface potential are detected when passing over the twilight zone and the Equatorial anomaly.

O-2 density and temperature profiles retrieving from direct solar Lyman-alpha radiation measurements

The resonance transition 2P-2S of the atomic hydrogen (Lyman-alpha emission) is the strongest and most conspicuous feature in the solar EUV spectrum. The Lyman-alpha radiation transfer depends on the resonance scattering from the hydrogen atoms in the atmosphere and on the O2 absorption. Since the Lyman-alpha extinction in the atmosphere is a measure for the column density of the oxygen molecules, the atmospheric O2 density and temperature profiles can be calculated thereof. A detector of solar Lyman-alpha radiation was manufactured in the Stara Zagora Department of the Solar-Terrestrial Influences Laboratory (STIL). Its basic part is an ionization camera, filled in with NO. A 60 V power supply is applied to the chamber. The produced photoelectric current from the sensor is fed to a two-channel amplifier, providing analog signal. The characteristics of the Lyman-alpha detector were studied. It passed successfully all tests and the results showed that the so-designed instrument could be used in rocket experiments to measure the Lymanalpha flux. From the measurements of the detector, the Lyman-alpha vertical profile can be obtained. Programs are created to compute the O2 density, atmospheric power and temperature profiles based on Lymanalpha data. The detector design appertained to ASLAF project (Attenuation of the Solar Lyman-Alpha Flux), a scientific cooperation between STIL—Bul.Acad.Sci., Stara Zagora Department and the Atmospheric Physics Group at the Department of Meteorology (MISU), Stockholm University, Sweden. The joint project was part of the rocket experiment HotPay I, in the ALOMAR eARI Project, EU’s 6th Framework Programme, Andøya Rocket Range, Andenes, Norway. The project is partly financed by the Bulgarian Ministry of Science and Education.

Main ionospheric trough studied from intercosmos-Bulgaria-1300

Abstract The main ionospheric trough is a phenomenon in the mid and high latitude ionosphere, characterized by an abrupt decrease of the electron and ion density and increase of the electron temperature. We here examine the behaviour of the trough for different geomagnetic conditions based on data from the Intercosmos-Bulgaria-1300 satellite. The dependence of the polar trough wall on the boundary of the soft electron precipitation from the plasma layer is shown, and a possible universal time dependence is examined.