Times Of Low Solar Activity Research Articles

High-resolution temperature and precipitation variability of southwest Anatolia since 1730 CE from Lake Gölcük sedimentary records

We report high-resolution multiproxy analyses [lithology, μXRF and magnetic susceptibility (MS)] of two short gravity sediment cores from the crater Lake Gölcük, southwest Turkey. Our results provide a detailed hydroclimatic record for the last ~290 years. Aided with factor analysis of μXRF data and 210210Pb and 137Cs dating, our multiproxy data show that the Lake Gölcük records documented a series of wet and dry periods between ~ 1730 (±71) and ~ 2018 (±3) CE. Wet periods are evidenced by dark olive green mixed lithology (sandy, clay, and silts) and high values in MS and log(Sr/Ca). On the other hand, dry periods are associated with light olive green clayey mud lithology and high values in log(Ca/K). We relate the wet periods to negative North Atlantic Oscillation (NAO-) and the dry periods to NAO+. Additionally, all wet periods are related with time of low solar activity and dry periods, except Dalton Minimum, are related with periods of high solar activity. Consequently, we suggest that hydroclimatic changes observed in the Lake Gölcük sedimentary records were caused by the influence of large-scale atmospheric circulation and solar activity.

Interaction of Space Weather Phenomena with Mars Plasma Environment During Solar Minimum 23/24

AbstractWe study the interaction of three solar wind (SW) structures, two stream interaction regions, and one interplanetary coronal mass ejection, with Mars’ plasma environment during November 20–27, 2007. This period corresponds to the solar minimum between the solar cycles 23 and 24 which was characterized by very low values of the SW density and dynamic pressure and low interplanetary magnetic field magnitude. During that time the Mars Express orbit was in the terminator plane, while the Earth, Sun, and Mars were almost aligned, so we use the Advance Composition Explorer and Solar Terrestrial Relations Observatory probes as SW monitors in order to identify and characterize the structures that later hit Mars. We find that the passage of these structures caused strong variations in the bow shock location (between 2.2 and 3.0 RM), compression of the magnetospheric cavity (up to 45%), and an increased transterminator flow below 2 RM (by a factor of ≤ 8). This study shows that during times of low solar activity, modest space weather phenomena may cause large variations of plasma flow at Mars.

Effects of Solar Activity on Ionospheric Ion Upflow During Geomagnetic Quiet Periods: DMSP Observations

Abstract Based on the Defense Meteorological Satellite Program (DMSP) observations during Solar Cycle 23, this paper examines solar activity dependence of ionospheric bulk ion upflow events (IUEs) in the Southern Hemisphere (SH). Much previous similar work was conducted over the Northern Hemisphere (NH) with measurements from European Incoherent Scatter (EISCAT). To eliminate the influence of geomagnetic disturbance on IUEs, we pick out observations during geomagnetic quiet periods (with Kp ≤ 2+). Results show that, ion upward densities and fluxes are dramatically elevated at times of high solar activity (HSA) but ion upward drifts and occurrences are increased at times of low solar activity (LSA) in the SH, which is consistent with the situation in the NH. The ratios between HSA and LSA for these four parameters (IUEs’ density, flux, upward drift and occurrence) are ~2.71, ~1.98, ~0.76 and ~0.57, respectively. Furthermore, lower flux event takes place frequently at LSA as the background ion density is low but the upward drift is large, while higher flux event happens commonly at times of HSA accompanied by high ion density but low upward velocity. Quantitatively, an increase in unit of solar activity (characterized by P index) causes a 4.2×108 m−3 increase in ion density and a 1.2×1011 m−2·s−1 enhancement in upward flux, together with a 0.6 m·s−1 and 0.02 % decrease in ion upward velocity and uprate, respectively. The acceleration from the ambipolar electric field is thought to be a possible mechanism affecting the dependence of IUEs on solar variations. For HSA, the acceleration from the ambipolar electric field weakens, but a large number of background ions provide abundant seeds for acceleration and upflow, which maintains a high IUE flux. It is inferred that upflow events and upward drifts are inhibited by the enhanced ionospheric background density.

Influence of solar activity changes on European rainfall

European hydroclimate shows a high degree of variability on every time scale. The variability is controlled by natural processes such as Atlantic ocean cycles, changes in solar activity, volcanic eruptions and anthropogenic factors. This contribution concentrates on the solar influence on European precipitation, a relationship which has been documented by a large body of published case studies. Here we are concentrating on the period 1901–2015 for which we compare sunspot data with monthly precipitation series of 39 European countries by calculating Pearson correlation coefficients for a multi-year cross-correlation window. The coefficients have been mapped out across Europe with the aim to identify areas in which solar activity may have influenced precipitation. Results show that February precipitation in Central and Western Europe yields the strongest solar response with coefficients reaching up to +0.61. Rainfall in June–July is equally co-driven by solar activity changes, whereby the solar-influenced zone of rainfall shifts from the British Isles towards Eastern Europe during the course of summer. Other months with noteworthy solar responses are April, May and December. On a decadal scale, the correlation between precipitation and solar activity in central Europe appears to be mostly positive, both statistically and by visual curve comparison. Yet, best positive correlations coefficients of February, June, July and December are typically reached when the solar signal lags rainfall by 1.5–2 years. Taking into account cause and effect, it is suspected that increases in Central European rainfall are actually triggered by the solar minimum some 3–4 years before the rainfall month, rather than the lagging solar maximum. Similar lags of a few years occur between solar activity and the solar-synchronized North Atlantic Oscillation (NAO) due to memory effects in the Atlantic. The literature review demonstrates that most multidecadal studies from Central Europe encountered a negative correlation between solar activity and rainfall, probably because short time lags of a few years are negligible on timescales beyond the 11 year solar Schwabe cycle. Flood frequency typically increases during times of low solar activity associated with NAO- conditions and more frequent blocking.

Continued decline of South Pole neutron monitor counting rate

AbstractThe counting rate of a neutron monitor at South Pole, Antarctica, displays a long‐term decline over the 49 year span from March 1964 to the present. The counting rate follows an 11 year cycle with maxima at times of low solar activity. However, after adjusting for the unusually high overall cosmic ray fluxes in 2009, we find that the 2009 peak rate (based on 27 day averages) was approximately 10% lower than the 1965 peak rate. This change is much larger than that recorded by any other neutron monitor. We suggest that the South Pole monitor, owing to its unique position at both high latitude and high altitude may have a hitherto unsuspected sensitivity to secular changes in the magnetic field of the Earth.

Global characteristics of the lunar tidal modulation of the equatorial electrojet derived from CHAMP observations

Abstract. It has been known since many decades that lunar tide has an influence on the strength of the equatorial electrojet (EEJ). There has, however, never been a comprehensive study of the tidal effect on a global scale. Based on the continuous magnetic field measurements by the CHAMP satellite over 10 years it is possible to investigate the various aspects of lunar effects on the EEJ. The EEJ intensity is enhanced around times when the moon is overhead or at the antipode. This effect is particularly strong around noon, shortly after new and full moon. The lunar tide manifests itself as a semi-diurnal wave that precesses through all local times within one lunar month. The largest tidal amplitudes are observed around December solstice and smallest around June solstice. The tidal wave crest lags behind the moon phase. During December this amounts to about 4 days while it is around 2 days during other times of the year. We have not found significant longitudinal variations of the lunar influence on the EEJ. When comparing the average EEJ amplitude at high solar activity with that during periods of solar minimum conditions a solar cycle dependence can be found, but the ratio between tidal amplitude and EEJ intensity stays the same. Actually, tidal signatures standout clearer during times of low solar activity. We suggest that the tidal variations are caused by a current system added to the EEJ rather than by modulating the EEJ. Gravitational forcing of the lower atmosphere by the moon and the sun is assumed to be the driver of an upward propagating tidal wave. The larger tidal amplitudes around December solstice can be related to stratospheric warming events which seem to improve the conditions for upward propagation. The results described here have to large extent been presented as a Julius-Bartels Medal Lecture during the General Assembly 2011 of the European Geosciences Union.

INFLUENCE OF THE AMBIENT SOLAR WIND FLOW ON THE PROPAGATION BEHAVIOR OF INTERPLANETARY CORONAL MASS EJECTIONS

We study three CME/ICME events (2008 June 1-6, 2009 February 13-18, 2010 April 3-5) tracked from Sun to 1 AU in remote-sensing observations of STEREO Heliospheric Imagers and in situ plasma and magnetic field measurements. We focus on the ICME propagation in IP space that is governed by two forces, the propelling Lorentz force and the drag force. We address the question at which heliospheric distance range the drag becomes dominant and the CME gets adjusted to the solar wind flow. To this aim we analyze speed differences between ICMEs and the ambient solar wind flow as function of distance. The evolution of the ambient solar wind flow is derived from ENLIL 3D MHD model runs using different solar wind models, namely Wang-Sheeley-Arge (WSA) and MHD-Around-A-Sphere (MAS). Comparing the measured CME kinematics with the solar wind models we find that the CME speed gets adjusted to the solar wind speed at very different heliospheric distances in the three events under study: from below 30 Rs, to beyond 1 AU, depending on the CME and ambient solar wind characteristics. ENLIL can be used to derive important information about the overall structure of the background solar wind, providing more reliable results during times of low solar activity than during times of high solar activity. The results from this study enable us to get a better insight into the forces acting on CMEs over the IP space distance range, which is an important prerequisite in order to predict their 1 AU transit times.

Solar activity dependence of ion upflow in the polar ionosphere observed with the European Incoherent Scatter (EISCAT) Tromsø UHF radar

The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromsø UHF radar between 1984 and 2008. In agreement with other work we find that the upward ion flux is generally higher when solar activity is high than when it is low. Ion upflow events and also the upward velocity behave the opposite: they are more frequently seen and higher, respectively, at times of low solar activity. In any year about 30–40% ion upflow is accompanied by ∼500 K higher electron temperature than the background temperature at 400 km altitude. Electron and ion heating in connection with upflow is nearly twice as prevalent during high solar activity as it is at low activity. The acceleration of ions by pressure gradients and ambipolar electric field becomes larger when solar activity is low than when it is high. This variation of the average acceleration is caused by the different shapes of electron density profiles for low and high solar activities. Ions start to flow up at above 450 km altitude when solar activity was high, and lower, at 300–500 km altitude, at low solar activity. It is suggested that the solar activity influences long‐term variations of the ion upflow occurrence because it modulates the density of neutral particles, the formation of the F2 density peak, and ion‐neutral collision frequencies in the thermosphere and ionosphere.

Estimating the 27‐day and 11‐year solar cycle variations in tropical upper stratospheric ozone

Spectral analysis of the solar backscatter ultraviolet (SBUV) satellite instrument ozone and solar flux time series was used to estimate the ozone response in the tropical upper stratosphere to the solar flux harmonics of the 27‐day Sun rotation cycle with periods of 9, 13.5, and 27 days. Solar UV flux at 205 nm, Mg II index, composite solar Lyman alpha, and 10.7 cm solar flux data sets were tested as proxies for the solar signal. The Mg II index has the highest coherency with tropical ozone in the upper stratosphere among all solar proxies. The analysis shows that during the periods of high solar activity, about half of the ozone variance for periods of 13.5 and 27 days near 40 km can be attributed to the fluctuations of the Mg II index. During the periods of low solar activity, the 27‐day signal is below the 90% statistical significance level, while the 13.5‐day signal is usually significant. Also, a 9‐day period can be seen in ozone data during the time of low solar activity. The ozone response to solar variations with a 27‐day period was used to estimate the 11‐year solar cycle amplitude in tropical ozone. Biases in individual SBUV instrument data were included as a part of the statistical model. In that case, the estimate from the 27‐day period minimum to maximum range of the 11‐year cycle is about 2%. This agrees with the data in layers 8–9 (38–43 km) derived from the SBUV data set. Below these layers, the amplitude of the 11‐year cycle is only about one third of the one estimated from the 27‐day cycle. Above these layers, the amplitude of the 11‐year cycle is larger than that estimated from the 27‐day cycle, but it is primarily due to the high amplitude of the 11‐year cycle in the Nimbus 7 SBUV data.

Long-term decline of South Pole neutron rates

The count rate recorded by a neutron monitor at South Pole, Antarctica, displays a long‐term decline over the 32‐year span from 1965 to 1997. The neutron rate follows an 11‐year cycle with maxima at times of low solar activity, but the 1997 peak rate was approximately 8% lower than the 1965 peak rate based on 27‐d averages. This change is much larger than that recorded by any other neutron monitor. We suggest that the South Pole monitor, owing to its unique position at both high latitude and high altitude (2820 m), has enhanced sensitivity at ∼1–3 GV relative to a sea level monitor and may be responding to a change in the intensity of primary cosmic rays in this rigidity region. Measurements of cosmic rays made aboard stratospheric balloons and on the IMP‐8 spacecraft support the possibility of a long‐term change in cosmic ray intensity.

Changes in the cosmic ray and heliomagnetic components of space climate, 1428–2005, including the variable occurrence of solar energetic particle events

The cosmic-ray record has been used to study the variations in the space climate, 1428–2005. Inversion of the data shows that the heliomagnetic field (HMF) near Earth increased steadily over the past 580 years, exhibiting the strongest fields during the last 50 years. The estimates indicate that the HMF at sunspot minimum has exhibited steadily increasing “floors” between the several Grand Minima, similar to the one evident in satellite data since 1965. The cosmogenic data for the past 10,000 years contain an ∼2300 year periodicity, and it is proposed that the increasing HMF strength since the 15th century represents the first quarter cycle of a ∼2300 year quasi-periodicity. It is concluded that the 11-year average total magnetic flux of the Sun has increased by a factor of ∼4.5 over the past 580 years. It is speculated that this could indicate a factor of ∼9 variation over the ∼2300 year cycle. The cosmic ray data and theoretical considerations show that the 22-year periodicity in the cosmic radiation flux at Earth was more dominant at times of low solar activity, compared to the present epoch. Comparison of the occurrence of solar energetic particle events, and the estimated HMF, shows that a substantial decrease in the size and frequency of occurrence of GLE (ground level enhancements) after 1958 coincides with a substantial increase in the HMF. This is consistent with the conclusion of [McCracken, K.G., Dreschhoff, G.A.M., Smart, D.F., et al. A study of the occurrence of large-fluence solar proton events and the strength of the interplanetary magnetic field. Solar Phys., 224, 359–372, 2004.] that lower values of the solar magnetic fields result in increased values of the Alfven Mach number, and thence to more efficient acceleration of solar cosmic rays prior to 1958. It is suggested that the variability of the solar and heliospheric magnetic fields may have introduced long term changes into the nature of geomagnetic phenomena; particle acceleration throughout the heliosphere and heliosheath; and possibly to the luminosity of the facular network of the Sun.

3D Simulations of the Solar Corona using Octree Compression

We present a new powerful tool to simulate the streamer belt of the solar corona based on forward modeling. It takes into account the temporal evolution of the corona and provides both qualitative and quantitative results. Starting from the National Solar Observatory photospheric magnetograms, the position of the neutral line at the source surface (2.5 Rsun) is caculated using the potential field source surface model. The plasma sheet of the streamer belt is centered around the current sheet represented as the radial extension of the neutral line. The 3D electron density is represented with octree compression and the radiance images are computed by a ray-tracing algorithm implementing the Thomson scattering. A multi-octree method allows to simulate the temporal evolution of the streamer belt and to compute the synoptic maps from time-series of generated images. The comparison between the synoptic maps of the streamer belt obtained with the SOHO/LASCO-C2 coronagraph and the simulated synoptic maps constructed from our model shows a global agreement for both radiance profiles and global behaviour of the streamer and confirms earlier findings by Wang et al. (1997) that the streamers are associated with folds in the plasma sheet. However, some features cannot be explained using this method and are interpreted by introducing two types of large-scale structures. Our results suggest that the potential field source surface model is not fully adequate for the description of the fine structure of the streamer belt, even during the time of low solar activity. We present new applications of our method to future coronographic observations with SECCHI/COR-2 on STEREO and SILC on Solar Orbiter.

On the 3-dimensional structure of the streamer belt of the solar corona

We investigate the three-dimensional structure of the streamer belt of the solar corona using a model that allows us to simulate its quasi-stationary configuration. Starting from the National Solar Observatory photospheric magnetograms, the position of the neutral line at the source surface () is determined using the potential field source surface model. A plasma sheet is centered around the current sheet represented as the radial extension of the neutral line. Comparing the synoptic maps of the streamer belt obtained with the SOHO/LASCO-C2 coronagraph and the simulated synoptic maps constructed from our model of the warped plasma sheet, we confirm earlier findings by Wang et al. (1997, ApJ, 485, 875; Wang et al. 2000, Geophys. Res. Lett., 27, 149) that the streamers are associated with folds in the plasma sheet. Although the large-scale structure of the streamer belt is described reasonably well, some features, however, cannot be explained in this framework. We propose that two types of large-scale structures take part in the formation of these additional features. The first one is an additional fold of the neutral line, which does not appear in the modeled source surface neutral line, but is well visible in photospheric magnetograms. The second one is a plasma sheet with a ramification in the form of a secondary short plasma sheet. We show that these structures better describe the observed configurations of the streamer belt. The secondary plasma sheet can be formed between two secondary current sheets connected to the main current sheet. Our results suggest that the potential field source surface model is not fully adequate for the description of the fine structure of the streamer belt, even during the time of low solar activity.

A Study of the Frequency of Occurrence of Large-Fluence Solar Proton Events and the Strength of the Interplanetary Magnetic Field

It has been shown previously that the number of very-large-fluence solar proton events inferred for the period since 1561 were more frequent at times of low solar activity (e.g., following the recovery from the Maunder minimum), than in the present epoch of high solar activity. An inverse dependence is demonstrated between the probability of observation of the very large-fluence solar proton events and the strength of the interplanetary magnetic field derived from empirical predictions. Using the observed dependence, it is predicted and demonstrated that large-fluence solar proton events have been observed at Earth more frequently near the recurrent minima of the solar activity cycle in the past than during the present epoch. We show that these results are explicable in terms of the linear dependence of the Alfven velocity upon the strength of the interplanetary magnetic field, leading to higher shock compression ratios in the past. These results indicate that this aspect of “solar weather” will be significantly influenced by the prevailing strength of the interplanetary magnetic field, and that recurrence of solar conditions similar to those of the solar activity minimum of solar cycles 12–14 (1878.9–1913.6) would be accompanied by a factor of ∼4 increase in the occurrence of large-fluence solar proton events.

Analytic solution of GPS atmospheric sounding refraction angles

The nonlinear system of equations for solving GPS atmospheric sounding’s bending angles are normally solved using Newton’s method. Because of the nonlinear nature of the equations, Newton’s method applies linearization and iterations. The method assumes the refraction angle to be small enough such that the dependency of the doppler shift on these angles are linear. The bending angles are then solved iteratively. Since the approach assumes the dependency of doppler shift on bending angles to be linear, which in actual sense is not, some small nonlinearity error is incurred. The Newton’s iterative method is often used owing to the bottleneck of solving in exact form the nonlinear system of equations for bending angles. By converting this system of trigonometric nonlinear equations into algebraic, the present contribution proposes an analytic (algebraic) algorithm for solving the bending angles and presents the geometry of the solution space. The algorithm is tested by computing bending angles of three CHAMP occultation data and the results compared to those of iterative Newton’s approach. Occultation 133 of 3rd May 2002 is selected as it occurred during diurnal solar radiation maximum past afternoon. During this time, the effect of ionospheric noise is high. Occultations number 3 of 14th May 2001 and number 6 of 2nd February 2002 were selected since they occurred past mid-night, a time of low solar activity and thus less effect of ionospheric noise. The results for occultation 133 of 3rd May 2002 indicate that the nonlinearity errors in bending angles increase with decrease in height to a maximum absolute value of 0.00069 ◦ (0.1%) for the region 5–40 km during period of high solar activity. Such nonlinearity errors are shown to impact significantly on the computed impact parameters to which the bending angles are referred. During low solar activity period, the nonlinearity error was relatively small for occultation number 3 of 14th May 2001 with maximum absolute value of 0.00001 ◦ . The analytical algorithm

The three‐dimensional solar wind at solar activity minimum

In early 1998 the Ulysses spacecraft made history by completing an orbit around the Sun inclined to the solar equator by 80°. It observed the properties of the high‐latitude heliosphere at a time of low solar activity when the configuration of the solar magnetic field was relatively simple. The solar wind data acquired by Ulysses have placed firm observational constraints on theories of the acceleration of the fast wind from the polar regions. The speed of the polar solar wind was in the range 750–800 km s−1, with a slow increase toward the poles. The polar proton flux of ∼2 × 108 cm−2 s−1 was only two‐thirds the low‐latitude flux but still sufficiently high to require energy sources in addition to the conduction of heat from the hot solar corona. In comparison with the slow, low‐latitude solar wind, the heavy ions in the fast, high‐latitude wind had less elemental fractionation relative to the solar surface and a lower state of ionization. Fine structures in the polar solar wind, named microstreams, exhibited a correlation between speed, proton temperature, and alpha particle abundance. The absence of a latitude dependence of the widths of the microstreams suggests that they are caused by temporal, rather than purely spatial variations at the Sun. Magnetic field observations revealed a latitude‐independent radial field strength of ∼3 nT (normalized to 1 AU) and a large flux of outward propagating Alivén waves. The most probable direction of the interplanetary magnetic field was close to the Archimedes (Parker) spiral predicted on the basis of radial fields close to the Sun combined with the effects of solar rotation. The fluctuations about the average direction were so large, however, that there was little latitudinal variation of the average value of |BR|B, where BR is the radial component of the field with magnitude B. Latitudinal transport of field lines and nonradial field directions near the Sun appear to be other necessary additions to the simple Parker model. Some unexpected differences were found between the low‐ and the high‐latitude properties of the plasma from transient events called coronal mass ejections (CMEs). At high latitudes, the speeds of the CME plasma clouds were close to the speed of the ambient fast wind, shocks were observed both ahead of and behind the CMEs, and enhancements of the helium abundance and of high charge states of heavy ions were not observed, as they often are at low latitudes. The review closes with a summary of arguments in support of the view that the fast solar wind observed by Ulysses at intermediate latitudes has its origin at higher latitudes in the polar coronal holes.

An Evaluation of Jacchia 71 and MSIS90 Atmosphere Models with NASA ODERACS Decay Data

The Space Shuttle Discovery (STS-60) deployed six Orbital Debris Radar Calibration Spheres (ODERACS 1) on February 9, 1994. A year later on February 3, 1995, three more spheres and three small dipoles (ODERACS 2) were placed in orbit from Discovery (STS-63). The orbital lifetimes of these metal spheres ranged from 8 to 13 months. The continuous NORAD 2-line orbital elements and the known dimension and mass of each sphere provided a good opportunity for the evaluation of atmospheric density models. This paper presents the results of the evaluation of two commonly used atmospheric density models, Jacchia 71 and MSIS90, based on decay histories of these spheres. The NORAD 2-line elements were processed by a semi-analytical computer program LIFETIME, which differentially adjusts the ballistic coefficient used with one of the two atmosphere models, so that the computed orbit agrees with the observed orbit. The daily measurements of solar flux, F10.7, and geomagnetic index, Ap, were used by LIFETIME for atmosphere density prediction. Two sets of drag coefficient, Cd, were deduced from the ballistic coefficients for four altitude bins covering an altitude range from 100 km to 350 km. Physical drag coefficients for spheres were compared with the observed drag coefficients deduced from the NASA data. Results of comparison strongly suggest that the Jacchia 71 density model is 9 to 22% too high in predicting density at the above mentioned altitude range at a time of low solar activity. The results of this study further support the estimated 15% error in Earth atmosphere density models.

Upper thermospheric temperatures at South Pole

Six years of upper thermospheric temperatures measured using 630 nm emission in the vicinity of South Pole Station (75° geomagnetic latitude) have been analyzed both on a daily and hourly averaged basis to compare observed behavior with that predicted using the MSIS86 semi-empirical atmospheric model. Daily averaged temperatures are in good general agreement, but the response of the model to geomagnetic activity appears to be too weak to match the observed changes near the peak of the solar cycle. Observations at time of low solar activity showed that temperatures tend to be lower than predicted; on the other hand, temperatures were higher than predicted at high activity. Observed diurnal temperature variations are very similar in amplitude and phase compared to those predicted by MSIS86. Observed mean temperature gradients indicated that the maximum temperature in the polar cap could well be 200K higher than the maximum seen at South Pole Station and at a position geomagnetically higher in latitude. The highest observed temperatures are found on the geomagnetic nightside associated with the exit of the cross-polar thermospheric jet towards lower latitudes and the lowest temperatures are observed on the geomagnetic dayside in the region of inflow to the polar cap. No signature of cusp heating was found.

Recurrent geomagnetic disturbances and coronal holes as observed in Fe XIV λ5303 Å

We present data obtained with the Fe XIV λ5303‐Å coronal photometer of the Sacramento Peak Observatory for the period October 3 to December 22, 1976. During this period there was a pattern of three coronal holes and three emission regions near the solar equator. Recurrent geomagnetic disturbances were associated with two of these three holes. We propose a simple method for predicting recurrent geomagnetic disturbances at times of low solar activity based on coronal brightness and test it on our data. We show that the method gives correct results 72% of the time, compared with 64% for 27‐day recurrence.

Esro 4 model of global thermospheric composition and temperatures during times of low solar activity

We present an empirical model of global exospheric temperatures and densities of neutral constituents, based on data from 228 geomagnetically quiet days during the operation of the Esro 4 gas analyzer (mean Ap = 7). By means of an expansion in spherical harmonics the model infers exospheric temperatures from both the measured N2 and Ar densities. O and He densities are presented as extrapolated density distributions at 120 km altitude. The Esro 4 model complements the similar Ogo 6 mass spectrometer model with observations at considerably lower altitudes (240‐320 km) and lower levels of solar activity Inferred exospheric temperatures are typically 5‐9% below Ogo 6 model values for F10.7 = 120.