Changed relation between sunspot numbers,solar UV/EUV radiation and TSI during the declining phase of solar cycle 23

We study the mutual relation of sunspot numbers and several proxies of solar UV/EUV radiation, such as the F10.7 radio flux, the HeI 1083 nm equivalent width and the solar MgII core-to-wing ratio. It has been noted earlier that the relation between these solar activity parameters changed in 2001/2002, during a large enhancement of solar activity in the early declining phase of solar cycle 23. This enhancement (the secondary peak after the Gnevyshev gap) forms the maximum of solar UV/EUV parameters during solar cycle 23. We note that the changed mutual relation between sunspot numbers and UV/EUV proxies continues systematically during the whole declining phase of solar cycle 23, with the UV/EUV proxies attaining relatively larger values for the same sunspot number than during the several decennia prior to this time. We have also verified this evolution using the indirect solar UV/EUV proxy given by a globally averaged f₀(F2) frequency of the ionospheric F2 layer. We also note of a simultaneous, systematic change in the relation between the sunspot numbers and the total solar irradiance, which follow an exceptionally steep relation leading to a new minimum. Our results suggest that the reduction of sunspot magnetic fields (probably photospheric fields in general), started quite abruptly in 2001/2002. While these changes do not similarly affect the chromospheric UV/EUV emissions, the TSI suffers an even more dramatic reduction, which cannot be understood in terms of the photospheric field reduction only. However, the changes in TSI are seen to be simultaneous to those in sunspots, so most likely being due to the same ultimate cause.     

Influence of solar ultraviolet radiation intensity of 630 nm nightglow emission excitation

It is well known that the 630.0 nm nightglow emission intensity at mid-latitudes increased more than twofold in periods of maximum sunspot activity. It is assumed that this phenomenon is caused by variations in solar ultraviolet radiation intensity in solar activity cycles (Fishkova, 1983).     

Measurements of solar ultra violet radiation on the Tibetan Plateau and comparisons with discrete ordinate method simulations

Measurements of both broadband and spectral UV radiation have been carried out at Lhasa (29°40′N, 91°08′E, 3648 m above sea level) on the Tibetan Plateau, using a moderate bandwidth filter instrument (NILUV) and a Fixed Imaging Compact Spectrometer (FICS). In this paper, the erythemal UV dose rates deduced from broadband measurements during the period from 1 July 1996 to 10 December 1997 are presented. The observed highest erythemal UV dose rate is 500 (or 458) mW/m² in July 1996 (or 1997), and the corresponding daily erythemal UV dose can reach up to 7.60 (or 7.00) kJ/m² and 9.18 (or 8.96) kJ/m², respectively, for the monthly mean and the monthly maximum. Comparisons with the UV levels at other locations at similar latitudes show that both the monthly mean and monthly maximum erythemal UV doses at Lhasa can be higher by a factor of 1.3–1.5 than those at San Diego (32°05′N, 117°1′W) in summer (from May to August), and exceed the corresponding values at Perth (32°0′S, 115°8′E) in the southern hemisphere summer (from November to February) by a factor of 1.2–1.4. Comparisons of both the broadband measurements and spectral measurements with the outputs of a discrete ordinate method (DOM) radiative transfer model have also been conducted. The results from the comparisons of broadband measurements with model outputs show that a 15, 11 and 10% agreement may be achieved around solar noon (with solar zenith angle smaller than 60°), respectively, for global irradiances in the 305, 320 and 340 mm channels, whilst the corresponding agreements are about 8 and 4% for the erythemal UV dose rate and the 340–305 nm ratio, respectively. The comparisons of the measured spectral irradiance with model calculations indicate that large discrepancies may appear at wavelengths shorter than 310 nm and longer than 380 nm. However, a 10% agreement may be generally achieved in UVA for solar zenith angle lower than 55°C. The corresponding agreement is about 20 and 5%, respectively, for UVB and the erythemal UV dose rate.     

Variations in the 557.7-nm atmospheric emission during stratospheric warming events under conditions of high and low solar activity

The effect of anomalously high average nighttime intensities of the atomic oxygen 557.7-nm atmospheric emission (luminescence heights 85–115 km) during sudden winter stratospheric warming events (SWEs) in Eastern Siberia is considered. Analysis of the variations in the 557.7-nm emission intensity (I _557.7) revealed the interdaily I _557.7-nm variations during SWEs and high average monthly I _557.7-nm values in the winter months in conditions of high solar activity. It has finally been found that the variations with periods of several days, at a maximum of which anomalously high daily values of I _557.7 are observed, are superposed on the average I _557.7-level during SWEs at high solar activity. A high average level of I _557.7 in the winter months in Eastern Siberia can be related to the fact that the atomic oxygen concentration at altitudes of the 557.7 nm emission luminescence increases by a factor of 2–3 in years of high solar activity.     

Influence of meteorological and wave processes on the lower ionosphere during solar minimum conditions according to the data on midlatitude VLF-LF propagation

The statistical characteristics of the intensity of VLF-LF radio signals transmitted from the midlatitude radio stations and recorded by the receiver at the Mikhnevo geophysical observatory (54.94°N, 37.73°E; Institute of Geosphere Dynamics, Russian Academy of Sciences) in 2007–2010 are analyzed. The experiments revealed strong variations in the intensity of radio signals during the deep solar minimum conditions, when the medium does not experience impacts from above associated with solar and geomagnetic activity. We relate the observed variations to the disturbances from below, which are caused by the meteorological and wave processes occurring in the lower atmosphere.     

Variations of the Nightglow 557.7 nm Emission Intensity during Solar Cycle 23

Geomagnetism and Aeronomy - The results of photometric measurements of the intensity of 557.7 nm nightglow emission at a high geographical latitude of 63° N at the Yakutsk meridian (130°...     

Studies on the shift in the frequency of the first Schumann resonance mode during a solar proton event

The variation of the first Schumann resonance (SR) frequency spectra observed from the recorded data over Kolkata (22.56°N, 88.5°E) during a solar proton event (SPE) on July 14, 2000 has been presented. It shows increase in frequency during X-ray bursts and decrease during the period of occurrence of an SPE. The results from some other locations for the same event are also reported. The severe X-ray bursts recorded just before the proton event exhibit enhancement in frequency of the first mode due to enhancement of ionization in the D-region of the ionosphere. Some attempts are made to explain the observed variation during solar proton events in terms of the perturbations within the Earth–ionosphere waveguide on the basis of two-layer-model.     

SAR arc occurrence frequency during cycle 23 of solar activity

The occurrence frequency of SAR arcs during 1997–2006 has been analyzed based on the photometric observations at the Yakutsk meridian (Maimaga station, corrected geomagnetic coordinates: 57° N, 200° E). SAR arcs appeared in 114 cases (～500 h) during ～370 nights of observations (～3170 h). The occurrence frequency of SAR arcs increases to 27% during the growth phase of solar activity and has a clearly defined maximum ～36% at a decline of cycle 23. The SAR arc registration frequency corresponds to the variations in geomagnetic activity in this solar cycle. The dates, UT, and geomagnetic latitudes of photometric observations are presented for 1997–2006.     

Variations of the tropical O(1S) nightglow as observed with the Arecibo Observatory photometer and WINDII on UARS

This work investigates the tropical O(¹S) nightglow variations utilizing observations from the Wind Imaging Interferometer (WINDII) operated on the Upper Atmosphere Research Satellite (UARS) and photometers installed at the Arecibo Observatory (18°N, 67°W) in Puerto Rico. Both are long-term datasets, but there is limited overlap in the dates of observation. Their simultaneous observations of column integrated emission rate on 6 continuous days in 1993 have a consistent difference at some times. The ground-based emission rate includes the F-region component of the O(¹S) emission, while the vertical profiles of WINDII allow the E- and F-region emission to be separated. This allows subtraction of the F-region emission rate for the Arecibo measurements and leads to a decisive study of the O(¹S) nightglow variation. The monthly averaged integrated emission rates (excluding the F-region component) are computed for WINDII from November 1991 to August 1997 and for the photometer data through 1983–2004. The emission rates between WINDII and Arecibo exhibit a similar semiannual variation. They have maxima at the two equinoxes and minima at solstices with a larger maximum in spring than in fall, while the WINDII averaged emission rates are larger than the apparent photometer averages by a factor of 2 for the mean values. The satellite WINDII and ground-based Arecibo instruments sample local time hours differently, but the local time influence is found not to be the origin of the difference. Because the observation periods are for different solar cycle intervals for the WINDII and Arecibo datasets, the influence of the solar F10.7 cm flux on the O(¹S) nightglow emission rate is also studied. The comparison between WINDII and Arecibo further offers a possibility to assess the atmospheric transmittance for these ground-based observations. The MODTRAN estimated transmittance of 0.7 is reasonably consistent with the difference in the O(¹S) nightglow emission rate between WINDII and the Arecibo photometer.     

Seasonal features in the response of the mesopause temperature and emission intensities to solar activity variations

The seasonal dependences of the response of the hydroxyl ((6–2) band) and molecular oxygen O₂(b ¹Σ _g ⁺ ) ((0–1) band) emission intensities, temperature, and density indicator in the region of the hydroxyl emission maximum (87 km) to solar activity have been obtained based on the spectral observations of the mesopause emissions at Zvenigorod observatory during 2000–2007. The ratio of the OH (7–3) and (9–4) band intensities, characterizing the behavior of the vibrational temperature, has been used as an indicator of density. It has been established that the response of the studied mesopause characteristics to solar activity is positive in all seasons. In winter the response is maximal in the intensities and temperature and is minimal in the density indicator. The main mechanisms by which solar activity affects the mesopause characteristics have been considered. The behavior of the internal gravity waves with periods of 0.33–7 h depending on solar activity has been studied. It has been noted that these waves become more active at a minimum of the 11-year solar cycle.     

Global electron content during solar cycle 23

Global electron content (GEC) as a new ionospheric parameter was first proposed by Afraimovich et al. [2006]. GEC is equal to the total number of electrons in the near-Earth space. GEC better than local parameters reflects the global response to a change in solar activity. It has been indicated that, during solar cycle 23, the GEC dynamics followed similar variations in the solar UV irradiance and F _10.7 index, including the 11-year cycle and 27-day variations. The dynamics of the regional electron content (REC) has been considered for three belts: the equatorial belt and two midlatitude belts in the Northern and Southern hemispheres (±30° and 30°–65° geomagnetic latitudes, respectively). In contrast to GEC, the annual REC component is clearly defined for the northern and southern midlatitude belts; the REC amplitude is comparable with the amplitude of the seasonal variations in the Northern Hemisphere and exceeds this amplitude in the Southern Hemisphere by a factor of ～1.7. The dayside to nightside REC ratio, R(t), at the equator is a factor of 1.5 as low as such a GEC ratio, which indicates that the degree of nighttime ionization is higher, especially during the solar activity maximum. The pronounced annual cycle with the maximal R(t) value near 8.0 for the winter Southern Hemisphere and summer Northern Hemisphere is typical of midlatitudes.     

Comparison between IRI-2001 predictions and observed measurements of hmF2 over three high latitude stations during different solar activity periods

The monthly median values of the height of peak electron density of the F2-layer (hmF2) derived from ionosonde measurements at three high latitude stations, namely Narssarssuaq (NAR) (61.2 °N, 314.6 °E), Sondrestrom (SON) (67°N, 309.1°E) and College (COL) (69.9°N, 212.2°E) were analyzed and compared with the International Reference Ionosphere (IRI-2001) model, using Comité Consultatif International des Radio communications) (CCIR and Union Radio-Scientifique Internationale (URSI) options. The analysis covers hmF2 values for March Equinox (February, March, April), June Solstice (May, June, July), September Equinox (August, September, October), and December Solstice (November, December, January), during periods of high (2000–2001), medium (2004–2005) and low (2007–2008) solar activity. Generally, the IRI-2001 prediction follow fairly well the diurnal and seasonal variation patterns of the observed values of hmF2 at all the stations. However, IRI-2001 overestimates and underestimates hmF2 at different times of the day for all solar activity periods and in all the seasons considered. The percentage deviation never exceeded 20%, except during DEC SOLS at COL and SON and during MARCH EQUI at SON during low solar activity period. For all solar activity periods considered, both the URSI and CCIR options of the IRI-2001 model give hmF2 values close to the ones measured, but the URSI option performed better than the CCIR option.     

Behavior of the polar lower ionosphere during the solar flares in January 2005 according to the data of the partial reflections method

The results of the observations, using partial reflections, of the lower ionosphere over Tumannyi station in the Murmansk region (69.0° N, 35.7° E) during strong solar flares on January 15–20, 2005, are presented. The structure of the D region of the polar ionosphere and the effects of X-ray flares and fluxes of high energy protons on this region are considered. The anomalous values of electron density in the lower part of the D region, unusually low values of the height of the lower ionospheric boundary, complete or partial absorption of short and medium radiowaves, and bursts of the meter cosmic radio emission were detected during solar disturbances.     

Characterization of low latitude GPS-TEC during very low solar activity phase

We present the mean diurnal, seasonal and annual variations in TEC during the lowest solar activity phase from low latitude Indian zone recorded at Udaipur (Geog. Lat. 24.6°N, Geog. Long.73.7°E, Geomag. Lat. 15.6°N) using a GPS receiver. Seasonal variations in daytime TEC show a semiannual periodicity, with a minimum in winter. Results of seasonal variations have been compared with that of the IRI-2007 model. Model calculations reveal significant seasonal as well as longitudinal differences in TEC. Seasonal variations in the nighttime TEC reveal an annual periodicity. Near the crest of the EIA, TEC shows a very good correlation with the solar flux. The results also point to weakening of the anomaly crest as well as its spatial and temporal contraction with declining solar activity.     

Peculiarities of the magnetic flux emerging in the equatorial solar zone

The magnetic flux longitudinal distribution in the equatorial solar zone has been studied. The magnetic synoptic maps of the Wilcox Solar Observatory (WSO) during Carrington rotations (CRs) 2052–2068 in 2007 and early 2008 have been analyzed. The longitudinal distributions of the area of the zones where the photospheric magnetic field locally enhanced have been constructed for each CR. The obtained distributions indicate that the zones are located discretely and that a clearly defined one narrow longitudinal interval with the maximum flux is present. The longitudinal position of this maximum shifted discretely by ≈130° at an interval of 5.5 ± 0.5 CRs. A longitudinal shift of the zones with an increased magnetic flux multiple of 60° was observed between the hemispheres. In addition, a time shift of ≈2.5 CRs existed between the instants when the position of maximum fluxes in different hemispheres shifted. The established peculiarities of the magnetic flux longitudinal distribution and time dynamics are interpreted as an action of supergiant convection cells. These actions result in that magnetic fields are removed from the generation region through the channels that are formed between such cells at a longitudinal interval of 120°. The average synodic rotation velocity of the considered equatorial channels, through which the magnetic flux emerges, is 13.43° day^–1.     

Empirical model of variations in the emission of the Infrared Atmospheric system of molecular oxygen: 3. Temperature

Rotational temperatures of the 1.58-μm (0–1) band of the Infrared Atmospheric system of molecular oxygen (IRAO₂), measured in Zvenigorod (56° N, 36° E), are systematized and analyzed. An empirical dependence of variations in the temperature of the 1.58-μm emission on the solar zenith angle is derived. The use of parameters of the altitude distribution of emission intensity 1.27 μm of middle atmosphere temperature profiles, received from the AURA satellite, allowed for the study of daily variations in the temperature of the 1.58-μm emission. It is revealed that the behavior of these variations corresponds to daily variations in the atmospheric temperature at altitudes of the radiating layer of IRAO₂, received from the AURA satellite.     

Ionospheric total electron content and critical frequencies over Europe at solar minimum

The paper presents results obtained by analyzing high-resolution ionospheric vertical total electron content (vTEC) data set evaluated from a chain of European ground-based Global Positioning System (GPS) stations and its equivalent slab thickness, as well as the F2-layer critical frequency foF2 and propagation factor M(3000)F2 from nearby ionosonde stations over the period 2006–2007. The study covers data within an area between 36°N and 68°N geographic latitude, and 7°W and 21°E geographic longitude during these last two years of minimum solar activity in the 23rd solar cycle. It reveals 15 extraordinary events, all of which exhibited some form of large short-lived vTEC and foF2 enhancements of the duration of small-magnitude solar-terrestrial events. The results clearly show a well-defined vTEC and foF2 storm-like disturbance patterns developed under these conditions. They prove that there are still some open questions related to the large electron density variations during weak disturbances that require additional study for both their relevance to different Global Navigation Satellite Systems (GNSS) applications and their role in the formation and evolution of the daytime ionosphere at middle latitudes.     

Large-scale structure of the solar corona magnetic field

The configuration of the solar corona magnetic field has been studied. Data on the position of the K-corona emission polarization plane during the solar eclipses of September 21, 1941; February 25, 1952; and August 1, 2008, were used as an indicator of the magnetic field line orientation. Based on an analysis of these data, a conclusion has been made that the studied configuration has a large-scale organization in the form of a cellular structure with an alternating field reversal. The estimated cell size was 61° ± 6° (or 36° ± 2°) in longitude with a latitudinal extension of 40°?C50° in the range of visible distances 1.3?C2.0 R _Sun . A comparison of the detected cellular structure of the coronal magnetic field with synoptic {ie908-1} maps indicated that the structure latitudinal boundaries vary insignificantly within 1.1?C2.0 R _Sun . The possible causes of the formation of the magnetic field large-scale cellular configuration in the corona and the conditions for the transformation of this configuration into a two-sector structure are discussed.