Time Of Solar Minimum Research Articles

The Solar Wind Speed Expansion Factor [${v}$–$f _{\text{s}}$] Relationship at the Inner Boundary (18 $\text{R}_{\odot }$) of the Heliosphere

The accuracy of data-driven magnetohydrodynamics (MHD) models depends on accurate boundary conditions specified at the inner heliosphere. However, not all of the MHD parameters [$\boldsymbol{B}, \boldsymbol{v}, \rho , T$] are measurable close to the Sun at the present time, except the vector magnetic field [$\boldsymbol{B} $] at the photosphere. The solar wind speed [$\boldsymbol{v}$], which is probably most relevant to space-weather forecasting, is often modeled by the standard Wang–Sheeley (WS) formula, which is based on an inverse relationship between the solar wind speed [$\boldsymbol{v}$] at 1 AU and the expansion factor [$f_{\text{s}}$] estimated at 2.5 solar radii [$\text{R}_{ \odot }$], with the following generic form: ${v} = {v}_{1} + {v}_{2} f_{\text{s}}^{- \alpha }$ (where $\boldsymbol{v}$ is the solar wind speed at 18 $\text{R}_{\odot }$, $f_{\text{s}}$ is the magnetic-field expansion factor, and ${v}_{1}$, ${v}_{2}$, and $\alpha $ are three free parameters to be determined). While the WS formula uses “source projection” to determine the solar wind source, it does not treat the solar wind as plasma because it uses the solar wind speed observed at 1 AU to derive the empirical relationship. Thus, the resulting formula ignores the transport and acceleration of the solar wind as it propagates out into the heliosphere. The purpose of this study is to rectify this omission by using a numerical MHD simulation to find the optimal set of free parameters that relate the magnetic properties at the source surface to the plasma parameters at 1 AU. In addition to the expansion factor, conservation of mass [$\rho \boldsymbol{v}$], magnetic flux [$r ^{2} B$], and total pressure along the stream line are assumed to obtain the solar wind mass density, magnetic field, and temperature at 18 $\text{R}_{\odot }$. These parameters are used as the inner boundary conditions of our global three-dimensional MHD (G3DMHD) code to simulate solar wind plasma and field parameters out to ${\approx}\, 1~\mbox{AU}$. The simulation results are compared with the in-situ data from Wind to assess the accuracy. Such a procedure is repeated (880 times) to cover the three parameter regimes ($100 < {v}_{1} < 350~\mbox{km}\,\mbox{s}^{-1}$; $250 < {v}_{2} < 700~\mbox{km}\,\mbox{s}^{-1}$; and $0.2 < \alpha < 0.9$) to find the optimal set. The simulation is performed for the period of CR2082 [30 March 2019 to 27 April 2009]. It is found that $\boldsymbol{v} = 189 + 679 f_{\text{s}}^{-0.7}$ is the best formula to relate the solar wind speed at 18 $\text{R}_{\odot }$ to the expansion factor. Strictly speaking, this formula is most applicable for solar equatorial regions and near the times of solar minimum when there are few coronal mass ejection events.

Hale cycle and long-term trend in variation of galactic cosmic rays related to solar rotation

Context. Galactic cosmic ray (GCR) intensities around solar minimum times are modulated by magnetic drifts that depend on the overall solar polarity. GCR intensities reach a higher but more narrow peak during negative minima than during positive minima. However, despite these higher intensities, the variation of GCRs over timescales of solar rotation is smaller during negative minima than during positive minima. Aims. We study the variation of GCR intensity over the 27-day synodic solar rotation and over the 14-day half-rotation, in particular the long-term trend and cyclic pattern of this variation, and propose a unifying explanation for the observations. Methods. We used two high-latitude neutron monitors, Oulu and Apatity, which are most sensitive to the low-energy part of the GCR spectrum and thereby more strongly affected by the changes in the conditions of the local heliosphere. We calculated the yearly mean amplitudes of the GCR intensity variation during the full solar rotation ( A 27 ) and half-rotation ( A 14 ) in 1964–2016. Results. We verify that the A 27 and A 14 amplitudes exhibit a clear 22-yr Hale cycle during solar minima at both stations, with larger amplitudes in positive minima. We find that the mean amplitude of the Hale cycle is about 30–45% of the mean amplitude for A 14 , while is only about 15–30% for A 27 . We also find that all amplitudes depict a declining long-term trend, which we suggest is due to the weakening of solar polar magnetic fields during the last four solar cycles and the ensuing latitudinal widening of the heliospheric current sheet (HCS) region. An exceptionally wide HCS region during the last solar minimum, when A 14 reached its all-time minimum, is demonstrated by Ulysses probe observations. Conclusions. Our results emphasize the effect of polarity-dependent drift and the properties of the HCS in modulating the variation of GCR intensity during solar rotation in solar minimum times. The second rotation harmonic yields a larger Hale amplitude than the first because it is more probable for the Earth to be outside the HCS only once during the rotation than twice or more, which more strongly reduces A 14 during negative polarity times than A 27 . With the HCS region widening from minimum to minimum, the decrease in A 14 is relatively faster than in A 27 .

Improved forecasting of thermospheric densities using multi-model ensembles

Abstract. This paper presents the first known application of multi-model ensembles to the forecasting of the thermosphere. A multi-model ensemble (MME) is a method for combining different, independent models. The main advantage of using an MME is to reduce the effect of model errors and bias, since it is expected that the model errors will, at least partly, cancel. The MME, with its reduced uncertainties, can then be used as the initial conditions in a physics-based thermosphere model for forecasting. This should increase the forecast skill since a reduction in the errors of the initial conditions of a model generally increases model skill. In this paper the Thermosphere–Ionosphere Electrodynamic General Circulation Model (TIE-GCM), the US Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Exosphere 2000 (NRLMSISE-00), and Global Ionosphere–Thermosphere Model (GITM) have been used to construct the MME. As well as comparisons between the MMEs and the “standard” runs of the model, the MME densities have been propagated forward in time using the TIE-GCM. It is shown that thermospheric forecasts of up to 6 h, using the MME, have a reduction in the root mean square error of greater than 60 %. The paper also highlights differences in model performance between times of solar minimum and maximum.

Systematically Asymmetric Heliospheric Magnetic Field: Evidence for a Quadrupole Mode and Non-Axisymmetry with Polarity Flip-Flops

Recent studies of the heliospheric magnetic field (HMF) have detected interesting, systematic hemispherical and longitudinal asymmetries which have a profound significance for the understanding of solar magnetic fields. The in situ HMF measurements since the 1960s show that the heliospheric current sheet (HCS) is systematically shifted (coned) southward during solar minimum times, leading to the concept of a bashful ballerina. While temporary shifts can be considerably larger, the average HCS shift (coning) angle is a few degrees, less than the 7.2∘ tilt of the solar rotation axis. Recent solar observations during the last two solar cycles verify these results and show that the magnetic areas in the northern solar hemisphere are larger and their intensity weaker than in the south during long intervals in the late declining to minimum phase. The multipole expansion reveals a strong quadrupole term which is oppositely directed to the dipole term. These results imply that the Sun has a symmetric quadrupole S0 dynamo mode that oscillates in phase with the dominant dipole A0 mode. Moreover, the heliospheric magnetic field has a strong tendency to produce solar tilts that are roughly opposite in longitudinal phase. This implies is a systematic longitudinal asymmetry and leads to a “flip-flop” type behaviour in the dominant HMF sector whose period is about 3.2 years. This agrees very well with the similar flip-flop period found recently in sunspots, as well as with the observed ratio of three between the activity cycle period and the flip-flop period of sun-like stars. Accordingly, these results require that the solar dynamo includes three modes, A0, S0 and a non-axisymmetric mode. Obviously, these results have a great impact on solar modelling.

Bashful ballerina: Southward shifted heliospheric current sheet

It is known since long [Rosenberg and Coleman, 1969] that one of the two sectors of the interplanetary magnetic field (IMF) observed at the Earth's orbit dominates at high heliographic latitudes during solar minimum times, reflecting the poloidal structure of the global solar magnetic field at these times. Here we find that while this latitudinal variation of the dominant IMF sector around the solar equator is valid for both solar hemispheres during the last four solar minima covered by direct observations, it is systematically more strongly developed in the northern heliographic hemisphere. This implies that the average heliospheric current sheet is shifted or coned southward during solar minimum times, suggesting that the temporary southward shift of the heliosheet found earlier by Ulysses observations in 1995 is a persistent pattern. This also implies that the open solar magnetic field is north‐south asymmetric at these times, suggesting that the solar dynamo has an asymmetric component. Accordingly, the Sun with the heliosheet is like a bashful ballerina who is repeatedly trying to push her excessively high flaring skirt downward. However, the effective shift at 1 AU is only a few degrees, allowing the Rosenberg‐Coleman rule to be valid, on an average, in both hemispheres during solar minima.

Bulk flow velocities in the solar corona

We present velocity estimates of bulk motions in the solar corona using data from the Large Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory spacecraft (SOHO). We describe a new technique which automatically provides a mass-weighted mean velocity profile in an entirely objective fashion without the need for individual event identification. A weighted velocity profile of this kind reflects the motion of the energetically dominant component of the coronal mass ejection (CME) mass spectrum and is of particular interest in consideration of the overall energy budget of the CME process. We consider the mean motion within three latitudinal bands centred at 0°, 20° and 40° over a one-year period around the time of solar minimum. We find terminal velocities within the LASCO field of around 300 km s−1 in all latitude bands but note a latitudinal dependence in CME evolution through the low corona prior to reaching these velocities. We find evidence that ejections in the equatorial zone undergo continuous acceleration whilst at higher latitudes a discrete burst of acceleration is seen to occur at around 4 R⊙ from the Sun's centre with relatively little acceleration thereafter. We also consider the energy deposition rates necessary to generate these profiles.

Radiation dose rates in Space Shuttle as a function of atmospheric density

Current models of the inner trapped belt describe the radiation environment at times of solar minimum and solar maximum, respectively. These two models were constructed using data acquired prior to 1970 during a small solar cycle, and no valid model for the past two high solar cycles exists. There is a clear need to accurately predict the radiation exposure of astronauts at all times between the solar minimum and solar maximum, not only on the short duration Space Shuttle flights, but on the longer term stay onboard the Mir orbital station and the planned International Space Station (ISS). An analysis of the trapped absorbed dose rate, D, at six fixed locations in the habitable volume of the Shuttle shows a power law relationship, D=Aρ − n , where ρ is the atmospheric density, ρ. The index, n, is weakly dependent on the shielding, decreasing as the average shielding increases. A better representation is provided by D=A tan −1 [( ξ− ξ c)/( ξ c− ξ m)], where ξ=ln( ρ), and A, ξ c, and ξ m are constants. ξ c is related to the atmospheric density near the altitude of atmospheric cutoff. These relationships hold over nearly four decades of density variation and throughout the solar cycle. This then provides a method of calculating absorbed dose rate at anytime in the solar cycle. These empirically derived relations were used to predict the dose rates for eleven Space Shuttle flights carried out since January 1997. The predictions are in excellent agreement with measured values. This method reduces the uncertainties of a factor of about 2 for the AP-8 MIN/MAX models to less than 30%.

Annual variation in near‐Earth solar wind speed: Evidence for persistent north‐south asymmetry related to solar magnetic polarity

We study the annual variation in solar wind speed at Earth's orbit and in geomagnetic activity since mid‐1960's. The two parameters depict a very similar annual variation during the whole period. Annual variation has maximum amplitude around sunspot minima. The phase of annual variation reverses soon after solar maxima, following the Sun's polarity reversal and indicating a new type of 22‐year periodicity. Stronger solar wind is found at or close to the Earth's highest northern (southern, resp.) heliographic latitudes during solar minima with a negative (positive) magnetic polarity. This implies an asymmetric SW speed distribution across heliographic equator such that the minimum speed region during solar minimum times is displaced away from heliographic equator towards the northern magnetic hemisphere. This may result e.g. from a systematically larger extension of polar coronal holes from the Sun's magnetic south pole toward solar equator. We exclude the earlier explanations proposed for annual variation, such as accumulation of small comets within 1 AU, or internal solar variation.

Cosmic‐Ray Modulation Parameters Derived from the Solar Diurnal Variation

From an analysis of the solar diurnal variation in Galactic cosmic rays with rigidities between 17 and 195 GV from 1957 to 1990, we have derived the product of the parallel mean free path and the radial density gradient () and the bidirectional latitudinal gradient indicator G|z|. We find the rigidity spectrum of the solar diurnal anisotropy averaged over the entire time span to have a spectral index of -0.1 ± 0.2 and an upper limiting rigidity of 100 ± 25 GV. The yearly averaged spectrum may depend on the heliospheric polarity state, having a slightly positive spectral index during the A > 0 epoch. We find that the parameter depends inversely on rigidity, more so during the A > 0 polarity state. Around solar minimum of this epoch, is also much lower than for corresponding years in the A < 0 polarity state. Combining the derived values of with results from previous studies of the radial gradient, we find evidence for the values of λ (at all the rigidities examined) having a variation related to the magnetic polarity state of the heliosphere. G|z| indicates that at rigidities up to 185 GV a bidirectional latitudinal gradient exists and changes direction following solar magnetic polarity reversals, in agreement with drift theories. This gradient attains its largest magnitudes around times of solar minimum and also appears to be larger during A < 0 polarity states.

Streamer belt structure at solar minima

We present plasma data from IMP 8 and Voyager 2 demonstrating the character of the current (1996) solar minimum and compare the current solar minimum to past solar minima. Voyager 2 data from 50 AU and 16°S heliolatitude show a gradual 130 km/s increase in solar wind velocity which reaches a plateau of 580 km/s for 75 days in late 1996. We suggest that this plateau, from which solar rotation signatures are absent, consists of solar wind which originated at the Sun at the time of solar minimum. We find that the current solar minimum closely resembles that of the 1976 solar minimum; both are characterized by relatively thick streamer belts. During the 1986–87 solar minimum the streamer belt was only half as thick (∼8°) as in 1976 and 1996. This suggests a possible 22‐year cycle in solar wind characteristics and we predict that at the next solar minimum in about 2007 the streamer belt will again be very thin.

First Results from Swan Lyman α Solar Wind Mapper on SOHO

After one year of almost flawless operation on board the SOHO spacecraft poised at L1 Lagrange point, we report the main features of SWAN observations. SWAN is mainly dedicated to the monitoring of the latitude distribution of the solar wind by the La method. Maps of sky Lα emissions were recorded througout the year. The region of maximum emission, located in the upwind hemisphere, deviates strongly from the pattern that could be expected from a solar wind constant with latitude. It is divided into two lobes by a depression aligned with the solar equatorial plane called the Lyα groove already noted in 1976 Prognoz data. The north lobe is much brighter than the south lobe. These two characteristics can be explained qualitatively by an enhanced ionization along the neutral sheet where the slow solar wind is concentrated, which results from the higher low-latitude solar wind mass flux as measured by Ulysses. The groove is the direct imprint on the sky of the enhanced carving by the slow solar wind, at this time of solar minimum, when the tilt angle of the neutral sheet is small. The question is still pending to predict what will happen with the ascending phase of the solar cycle. Observations of comets are briefly mentioned, with the ability of SWAN to monitor the H2O production of many comets. Operations of the instrument are briefly described, including some instrumental problems which could be solved by software modifications sent to the instrument.

Observations of a sky Lyman α groove related to enhanced solar wind mass flux in the neutral sheet

We report several observations of the Lα interplanetary emission recorded by a photometer flown in 1977 on board the soviet spacecraft Prognoz‐5. The Prognoz scans sampled emission in a plane perpendicular to the sun‐spacecraft line and also traversed the Lα maximum emission region (MER), a region centered a few AU from the Sun on the upstream interstellar‐flow axis. These scans reveal the existence of a 10% decrease in intensity centered near the ecliptic plane and about 30° wide. All the dips form a new feature of the interplanetary emission, a “groove” aligned approximately with the ecliptic plane. This groove is present only near the upwind direction, and is interpreted as the result of enhanced ionisation of interstellar H by charge‐exchange with the solar wind in a sheet of approximately 30° width in latitude around the average position of the neutral sheet, at this time of solar minimum.

The association of Coronal Mass Ejections during solar maximum times with the heliomagnetic current sheet

The distance distribution of Coronal Mass Ejections (CMEs) with respect to the Heliomagnetic Current Sheet is studied. The analysis is carried out for all CMEs observed by the Solar Maximum Mission Satellite, years 1980 and 1984–86. We foud that during solar minimum times the CMEs are better associated with the heliomagnetic equator than at solar maximum.

Pioneer Venus and IMP 8 observations of the latitude dependence of the solar wind

Solar wind speeds and magnetic field data from the Pioneer Venus orbiter (0.7 AU) and IMP 8 spacecraft (1 AU) are compared to infer the latitudinal structure of the solar wind in the inner heliosphere between 1984 and 1987. It is found that solar wind streams and the interplanetary magnetic field (IMF) exhibited an unusual configuration during the last solar minimum (1986‐1987). This configuration only appeared during a limited period around the time of solar minimum, and was similar to the configuration that appeared during the previous solar minimum in 1975. Prior to solar minimum, the solar wind speed did not appear to vary with heliographic latitude, and the same high‐speed streams appeared at both spacecraft even when they were widely separated. The IMF had a two‐sector structure at all latitudes. During solar minimum, this situation changed, and high‐speed streams were excluded from low (&lt;3°) latitudes, while the vicinity of the solar equator was characterized by slow solar wind and irregular variations in speed. The sector structure of the IMF also changed during solar minimum, and a single polarity predominated at latitudes greater than 4°. The onset of these changes appears to be related to a decrease in the inclination of the coronal neutral line; from approximately 40° at the end of 1984 to approximately 10° in the beginning of 1986. This is consistent with suggestions by previous observers that the coronal neutral line is associated with a sheet of low‐speed solar wind without high‐speed streams.

Mars mesosphere and thermosphere coupling: Semidiurnal tides

The National Center for Atmospheric Research thermosphere ionosphere general circulation model for Earth has been modified to examine the three‐dimensional structure and circulation of the upper atmosphere of Mars (MTGCM) (Bougher et al., 1988b, 1990). Recent examination of Mariner 9 UVS airglow measurements taken during a global dust storm provide evidence of large temperature variations and atomic oxygen distributions uncorrelated with solar activity and the corresponding MTGCM in situ driven winds. We suspect significant forcing of the thermosphere from below as a result of upward propagating gravity waves or tides generated by solar heating of airborne dust (Stewart et al., 1992). The effects of upward propagating tides are introduced into the MTGCM by appropriately specifying its lower boundary condition according to classical tidal theory. We initially adapt the terrestrial scheme used by Fesen et al. (1986) to a Mars model appropriate to Mariner 9 near‐solar‐minimum conditions. Estimates of the amplitude and phase of the likely dominant semidiurnal (2,2) mode at the mesopause (∼100 km) are specified for a range of possible lower atmosphere dust conditions. MTGCM simulations contrasting tidally driven fields with solar‐only forced ones show a dramatic change in the horizontal and vertical wind patterns, whereby the global temperature and oxygen distributions are also modified significantly. This semidiurnal component predominates below 135 km, while the in situ solar‐driven diurnal component is largely dominant above. Constructive interference serves to enhance midafternoon exospheric temperatures toward Mariner 9 observed values. The thermospheric response and the altitude of penetration of these semidiurnal tides is found to be much greater during solar minimum periods when dissipation due to viscosity and thermal conductivity is diminished from that at solar maximum. Finally, the Martian response during dusty periods is predicted to be much larger than that typically observed for Earth. Martian dust‐driven tides, especially during solar minimum time periods, cannot be ignored when addressing the Mars thermospheric structure and dynamics.

Solar cycle effects on the structure of the electron density profiles in the dayside ionosphere of Venus

Since 1975, United States and Soviet Venus‐orbiting spacecraft have carried out more than 190 radio occultation measurements of the vertical electron density structure of the dayside ionosphere of Venus: 148 of these have been produced by the Pioneer Venus orbiter between 1979 and 1989; Venera 9 and 10 provided 13 measurements in 1975; and Venera 15 and 16 made 31 measurements in 1983. These measurements were made at various times of the solar cycle, including solar maximum and solar minimum. It has therefore been possible to observe the changes in the electron density structure of the dayside ionosphere of Venus that were brought about by changing solar activity. Some of the resulting observations are as follows: (1) The ionopause height is generally low for values of the solar zenith angle below about 50° regardless of the phase in the solar cycle. (2) At solar maximum, and at times of intermediate solar activity, the ionopause height for solar zenith angles greater than about 50° is highly variable, ranging from a minimum of about 200 km to a maximum of more than 1000 km. (3) At times of solar minimum the great majority of all ionopause heights for all solar zenith angles are uniformly low, lying between 200 and 300 km, with the notable exception of three Venera 9 and 10 measurements. The compressed nature of the Venus ionosphere at solar minimum is most likely produced by permeation of the ionosphere by the solar wind magnetic field, which occurs when the solar wind dynamic pressure exceeds the ionospheric plasma pressure. This happens at times of solar maximum at low solar zenith angles and when the solar wind dynamic pressure is exceptionally high. At solar minimum the Venus ionosphere, like the Martian ionosphere at all phases of the solar cycle, does not appear to have sufficient thermal pressure to balance the incident solar wind pressure.

The physics of cosmic ray modulation: Heliospheric propagation during the 1987 Minimum

The period in and near the 1987 solar minimum of modulation is a unique interval in the 11‐year solar cycle modulation for determining whether both the heliospheric current sheet and charged particle gradient drift contribute significantly to the modulation of galactic cosmic rays and the anomalous components. Our investigation of this period is based on measurements from neutron monitors and the IMP 8 satellite at 1 AU and the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft distributed throughout the heliosphere to ∼46 AU. Three observations essential for the analysis are as follows: (1) the brief time‐intensity profile for cosmic ray minimum could be traced relatively unchanged from 1 AU to ∼41 AU at the average solar wind velocity, (2) the onset of enhanced modulation in the new solar cycle at 1 AU began at the highest magnetic rigidity and progressed in time over a period of months to particles of low magnetic rigidity, and (3) the average tilt angle of the neutral current sheet was available from Hoeksema (1989) as a function of time for the period centered on the 1987 solar minimum at 1 AU. To account for the observations, we propose a model for the onset of the new modulation cycle which includes a change in the average tilt angle of the current sheet as it propagates outward through the heliosphere at the solar wind velocity. Calculations based on our model with an inwardly directed north solar polar magnetic field (which prevailed in the 1985–1988 period) include diffusion, convection, adiabatic deceleration, and drift for the inward propagation of cosmic ray protons in the energy range from 20 to 1500 MeV and reproduced the rigidity phase lag effect observed in the 1987 data. On the other hand, for the reversed solar magnetic field polarity (e.g., 1954, 1977) the model calculations show that an enhanced onset for these solar cycles has a negligible rigidity phase lag at 1 AU, in agreement with the observed 1977 onset of modulation. We point out that transient phenomena (e.g., Forbush decreases and interaction regions) appear to dominate the higher levels of solar modulation following the 1987 period. A consequence of our analysis is that at the time of solar minimum there is a minimum in the amplitude of the 11‐year variation of the radial gradient of integral cosmic ray intensity which propagates outward at the solar wind velocity. Therefore the time dependence of the amplitude of the radial gradient is not the result of the inward‐outward motion of a modulation boundary.

Volcanic eruptions and solar activity

The historical record of large volcanic eruptions from 1500 to 1980, as contained in two recent eruption catalogs, is subjected to detailed time series analysis. Two weak, but probably statistically significant, periodicities of ∼11 and ∼80 years are detected. Both cycles appear to correlate with well‐known cycles of solar activity; the phasing is such that the frequency of volcanic eruptions increases (decreases) slightly around the times of solar minimum (maximum). The weak quasi‐biennial solar cycle is not obviously seen in the eruption data, nor are the two slow lunar tidal cycles of 8.85 and 18.6 years. Time series analysis of the volcanogenic acidities in a deep ice core from Greenland, covering the years 553–1972, reveals several very long periods that range from ∼80 to ∼350 years and are similar to the very slow solar cycles previously detected in auroral and carbon 14 records. Mechanisms to explain the Sun‐volcano link probably involve induced changes in the basic state of the atmosphere. Solar flares are believed to cause changes in atmospheric circulation patterns that abruptly alter the Earth's spin. The resulting jolt probably triggers small earthquakes which may temporarily relieve some of the stress in volcanic magma chambers, thereby weakening, postponing, or even aborting imminent large eruptions. In addition, decreased atmospheric precipitation around the years of solar maximum may cause a relative deficit of phreatomagmatic eruptions at those times.

Evidence for change in particle excitation of Jupiter's atmosphere 1968–1979

Analysis of the Pioneer 10 and rocket observations of disk averaged emission from the sunlit atmosphere of Jupiter indicates that the spectrally integrated EUV brightness was reduced by at least a factor of 2 relative to Voyager spacecraft observations in 1979. Most of the variation is caused by the HLyα component in the spectrum, which was reduced ≈ 1 order of magnitude near the time of solar minimum in 1972‐1973. Although the analysis of the data does not produce entirely consistent results, the weight of evidence points to a factor of order ≈ 2 lower abundance of H in Jupiter's atmosphere in 1972–1973 relative to 1979. The low emission rate in HLyα near the time of solar minimum in this proposed scenario is caused by an electroglow energy deposition rate reduced by a factor of ≈ 3. The apparent reduced abundance of H implies a reduced thermospheric temperature, even under the assumption of a constant electroglow deposition rate.

North-South asymmetry in sudden disappearances of solar prominences

This paper presents the results of a study of the N-S asymmetry in sudden disappearances (SD) of solar prominences during solar cycles 18–21, obtained as a part of a more extensive research on SD and reappearances during years 1931–1985 (Ballester, 1984). As can be seen, the N-S SD asymmetry curve is not in phase with the solar cycle and peaks about the time of solar minimum, the asymmetry reverses in sign during the solar maximum, being, this change of sign, coincident with the reversal of the Sun's magnetic dipole. The SD asymmetry curve can be fitted by a sinusoidal function with a period of eleven years. On the other hand, the SD asymmetry curve shows a strong coincidence with the N-S asymmetries presented by other solar activity manifestations as studied by different authors.