Minimum Of Solar Cycle Research Articles

High-speed streams from coronal holes and coronal mass ejections around the solar minimum of cycle 22

The soft X-ray telescope on board Yohkoh enables continuous observations of coronal holes and disturbances in the solar corona. Although there were recurrent high-speed streams emanated from the coronal holes around the solar minimum of cycle 22, their effect on geomagnetic disturbances seems to be weak during this period. A half of geomagnetic disturbances ( Dst≧−50 nT ) were associated with coronal mass ejections (CMEs). Several geomagnetic disturbances associated with both high-speed streams and CMEs had large magnitude and long duration.

Uniformity of Longitudinal Distribution of Coronal Mass Ejections

It was found that the heliographic longitudinal distribution of the occurrence of coronal mass ejections (CMEs) was not uniform around the solar minimum of cycle 21. This is confirmed by using the χ2 test for goodness of fit. Sunspot groups and flares show similar non-uniform distributions during the same period. This suggests that there was a longitudinal range where both large-scale and small-scale solar magnetic activities were very low around the solar minimum of cycle 21. The longitudinal range with low activity was more than 150 degrees and continued for approximately four rotations. As solar activity increased, CMEs occurred almost uniformly in heliographic longitude.

Solar EUV quasi-biannual variations

Problems arise with accurate assessment of the solar activity variations both during the solar EUV model derivation and satellite EUV observations analysis. The present day experimental accuracy of the EUV flux variations is around 30%. The indirect ionospheric method in special cases can provide better accuracy for EUV variations. Quasi-biannual variations of solar background EUV are revealed for the period of solar cycle minimum, the amplitude of such variations being around 20%.

An updated climatology of thermospheric neutral winds and F region ion drifts above Millstone Hill

Incoherent scatter radar data from 63 experiments during the period 1984–1997 have been used to update earlier published climatologies of ion drifts and neutral thermospheric winds at Millstone Hill. Data are binned according to season, solar activity, geomagnetic activity, and local solar time. The derived ion drift patterns are similar to those previously published, but we now find that the main effect of the increased magnetic activity is an enhancement of the usual nighttime west‐ward ion drift. A tidal decomposition of the winds from geomagnetically quiet conditions is carried out for each season/solar activity level to extract diurnal means, diurnal amplitudes and phases, and semidiurnal components. These winds are compared with previous results at Millstone Hill and the HWM‐93 model. Using our expanded wind database, statistically significant semidiurnal components are now found in both summer and winter, at both solar maximum and solar minimum. Our bin‐averaged results confirm earlier published findings that at all seasons the diurnal mean winds are more strongly equatorward and the diurnal amplitudes are stronger at solar cycle minimum than at solar cycle maximum. Diurnal amplitudes derived in the present work are larger than those found previously because we now use a smaller value for the O+, O collision frequency. Differences in the diurnal mean and diurnal amplitude between recent solar minimum data (1993–1997) and data from the previous solar minimum (1984–1986) are attributed to a difference in the EUV flux as inferred from Millstone Hill electron density data which one would not expect from the F10.7 index. This affects the ion drag and apparently also the relative importance of the EUV and high latitude heat sources.

What is solar cycle minimum?

Establishing the time of minimum between cycles 22 and 23 is complicated because there are two periods of low solar activity during 1996. To resolve this controversy, we studied the time of minimum in terms of the historical basis for defining this fiducial point in the solar cycle using several measures of solar activity, as well as the cycle membership of active regions observed during the minimum phase between these two cycles. Our conclusion is that cycle minimum is not defined solely on the basis of the occurrence of the minimum in the smoothed sunspot number, but rather by several additional parameters, including the monthly (or rotationally) averaged sunspot number, the number of regions (total, new‐ and old‐cycle), and the number of spotless days. Using these specific measures of solar activity, we recommend that the minimum between cycles 22 and 23 occurred in September 1996 (1996.7) and not in May 1996 (1996.4).

Short-term periodicities of the sun's ‘mean’ and differential rotation

We have looked for periodicities in solar differential rotation on time scales shorter than the 11-year solar cycle through the power- spectrum analysis of the differential rotation parameters determined from Mt. Wilson velocity data (1969–1994) and Greenwich sunspot group data (1879–1976). We represent the differential rotation by a set of Gegenbauer polynomials (ω(φ)= \(\bar A\) + \(\bar B\) (5sin2φ−1)+ \(\bar C\) (21sin4φ−14sin2φ+1)). For the Mt. Wilson data, we focus on observations obtained after 1981 due to the reduced instrumental noise and have binned the data into intervals of 19 days. We calculated annual averages for the sunspot data to reduce the uncertainty and corrected for outliers occuring during solar cycle minima. The power spectrum of the photospheric ‘mean rotation’ \(\bar A\) , determined from the velocity data during 1982–1994, shows peaks at the periods of 6.7–4.4 yr, 2.2 ± 0.4 yr, 1.2 ± 0.2 yr, and 243 ± 10 day with ≥99.9% confidence level, which are similar to periods found in other indicators of solar activity suggesting that they are of solar origin. However, this result has to be confirmed with other techniques and longer data sets. The 11-yr periodicity is insignificant or absent in \(\bar A\) . The power spectra of the differential rotation parameters \(\bar B\) and \(\bar C\) , determined from the same subset, show only the solar cycle period with a ≥99.9% confidence level.

Solar variability and climate change: Geomagnetic aa index and global surface temperature

During the past ∼120 years, Earth's surface temperature is correlated with both decadal averages and solar cycle minimum values of the geomagnetic aa index. The correlation with aa minimum values suggests the existence of a long‐term (low‐frequency) component of solar irradiance that underlies the 11‐year cyclic component. Extrapolating the aa‐temperature correlations to Maunder Minimum geomagnetic conditions implies that solar forcing can account for ∼50% or more of the estimated ∼0.7–1.5°C increase in global surface temperature since the second half of the 17th century. Our analysis is admittedly crude and ignores known contributors to climate change such as warming by anthropogenic greenhouse‐gases or cooling by volcanic aerosols. Nevertheless, the general similarity in the time‐variation of Earth's surface temperature and the low‐frequency or secular component of the aa index over the last ∼120 years supports other studies that indicate a more significant role for solar variability in climate change on decadal and century time‐scales than has previously been supposed. The most recent aa data for the current solar minimum suggest that the long‐term component of solar forcing will level off or decline during the coming solar cycle.

Solar UV Activity at Solar Cycle 21 and 22 Minimum from NOAA-9 SBUV/2 Data

Although solar ultraviolet (UV) irradiance measurements have been made regularly from satellite instruments for almost 20 years, only one complete solar cycle minimum has been observed during this period. Solar activity is currently moving through the minimum phase between cycles 22 and 23, so it is of interest to compare recent data taken from the NOAA-9 SBUV/2 instrument with data taken by the same instrument during the previous solar minimum in 1985–1986. NOAA-9 SBUV/2 is the first instrument to make continuous solar UV measurements for a complete solar cycle. Direct irradiance measurements (e.g., 205 nm) from NOAA-9 are currently useful for examining short-term variations, but have not been corrected for long-term instrument sensitivity changes. We use the Mgii proxy index to illustrate variability on solar cycle time scales, and to provide complementary information on short-term variability. Comparisons with contemporaneous data from Nimbus-7 SBUV (1985–1986) and UARS SUSIM (1994–1995) are used to validate the results obtained from the NOAA-9 data. Current short-term UV activity differs from the cycle 21–22 minimum. Continuous 13-day periodicity was observed from September 1994 to March 1995, a condition which has only been seen previously for shorter intervals during rising or maximum activity levels. The 205 nm irradiance and Mgii index are expected to track very closely on short time scales, but show differences in behavior during the minimum between cycles 22 and 23.

Geomagnetic activity effects on thermospheric tides: a compendium of theoretical predictions

The theoretical effects of auroral activity on thermospheric tides are investigated using simulations from the National Center for Atmospheric Research thermosphere-ionosphere general circulation model. Simulations for March, June, and December during solar cycle minimum and maximum are presented for three levels of geomagnetic activity representing quiet, moderate, and disturbed conditions. The mean, diurnal, and semidiurnal components of the model neutral temperatures and horizontal winds at geographic longitude 70 ° W at 17.5 ° N, 42.5 ° N, and 67.5 ° N are examined. The model predicts that the effects of varying geomagnetic activity are larger at solar cycle minimum than solar cycle maximum and increase with increasing latitude and altitude. In general, the predicted mean temperatures increase, the mean zonal winds become more westward and the mean meridional winds more equatorward as geomagnetic activity increases. In the upper thermosphere, the effects of geomagnetic activity on the mean and tidal neutral winds and temperatures are such that diurnal temperature amplitudes decrease, diurnal zonal winds generally decrease at low and middle latitudes and increase at high latitudes, and diurnal meridional winds increase; semidiurnal amplitudes generally increase. At low altitudes in high latitudes, increasing levels of geomagnetic activity cause tidal amplitudes to increase strongly, peaking near 140 km. At middle latitudes, the lower thermosphere diurnal winds increase in March and June and decrease in December. At low latitudes, the effects at low altitudes are generally small and complex. Comparison with observational and other theoretical results is inconclusive because of a lack of data.

Solar-Radius Variations Over a Solar Cycle Observed with the Tokyo Photoelectric Meridian Circle

AbstractAs the Sun shows the well-known magnetic activity of eleven-year period, many other interesting observational features of the Sun have been examined in connection with the solar cycle. The diameter of the Sun is one of these interesting observable quantities. The apparent radius of the Sun has been observed regularly since 1985 with the Tokyo Photoelectric Meridian Circle (Tokyo PMC) at Mitaka. Here, we show the average value of the solar radii observed by us. We also show that the annual-mean values of the observed radii take the smallest value near the solar-cycle maximum, in 1989 and 1990, and the largest ones around the solar-cycle minimum, in 1986 and 1994. The average peak-to-peak shift of the annual-mean solar radius for the solar cycle is ΔR⊙/R⊙ ~ 1.2 × 10−4, which is close to the relative change of the frequency of the 5-minute p-mode solar oscillation within one solar cycle.

Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

We have digitized the Ca ii K spectroheliograms, observed at the National Solar Observatory at Sacramento Peak, for the period 1980 (maximum of solar cycle 21), 1985 (minimum of solar cycle 21), 1987 (beginning of the ascending phase of solar cycle 22), 1988 and 1989 (ascending phase and maximum of solar cycle 22), and 1992 (declining phase of solar cycle 22). A new method for analyzing the K spectroheliograms has been developed and applied to the K images for the time interval of 1992. Using histograms of intensity, we have segregated and measured the cumulative intensity and area of various chromospheric features like the plages, magnetic network and intranetwork elements. Also, the full width at half maximum (FWHM) derived from the histograms has been introduced as a new index for describing the chromospheric activity in the K-line. The full-disk intensity (spatial K index) has been derived from spatially-resolved K images and compared to the spectral K index derived from the line profiles for the full disk. Both the spatial K index and FWHM have been compared to the UV irradiance measured in the Mg ii h and k lines by the NOAA9 satellite and found that they are highly correlated with the Mg ii h and k c/w ratio.

LET spectra of trapped anomalous cosmic rays in low-Earth orbit

Observations aboard Cosmos satellites discovered /1/ trapped anomalous cosmic rays (ACRs), tracked the variation in their intensity in 1986–1988, and measured their fluence, spectrum, and composition /2/ at solar minimum in the previous solar cycle. The MAST instrument aboard the SAMPEX satellite has observed trapped anomalous cosmic rays in the present solar cycle, confirmed the general features of the Cosmos data, and provided the first detailed observations of trapped ACRs /3,4,5/. In this paper we apply theoretical modeling of trapped ACRs, which is shown to provide a reasonably good description of both the Cosmos and SAMPEX data, to calculate the integral linear-energy-transfer (LET) spectra due to trapped ACRs in typical low-Earth orbits. We compare these calculations with the LET spectra produced by galactic cosmic rays (GCRs) and non-trapped ACRs in order to assess the relative radiation hazard posed by trapped ACRs.

Simulations of seasonal and geomagnetic activity effects at Saint Santin

Simulations from the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model constitute controlled numerical experiments which may be used to assess current understanding of Earth's upper atmosphere. Comparisons with a long term data base are particularly valuable in this regard. Accordingly model simulations of geomagnetically quiet and active periods are compared with an observational database from Saint Santin. The simulations and observations are for equinox and northern summer and winter during solar cycle minimum. The observations consist of the diurnal variation of the meridional neutral winds near 300 km; harmonic analysis yielded the mean components and the 24‐, 12‐, 8‐, and 6‐hour waves. The model/data comparisons for the diurnal variations are good to excellent: differences are generally ≤ 25 m/s with largest differences typically occurring between 1800 and 0600 UT. In the observations, the diurnal component is approximately 60 m/s in amplitude and 12 hours in phase during quiet periods. These values persist during active periods except in summer when the diurnal amplitude falls to 47 m/s. The model predicts a weaker diurnal amplitude in winter than the observations indicate; it also does not predict the observed decrease of the diurnal amplitude in summer with increasing activity. Harmonic analysis of the data indicated that (1) 12‐r and 8‐hour waves are important in summer and equinox; in winter the variation is largely diurnal; (2) the semidiurnal and terdiurnal waves vary with season during quiet periods; and (3) the largest effect of geomagnetic activity is in the 12‐ and 8‐hour waves. In the model, the semidiurnal and terdiurnal tides do not vary with season. Further, the effects of varying geomagnetic activity are predominantly in the diurnal component; the semidiurnal and terdiurnal tides in the TIGCM are relatively unaffected in marked contrast to the observations. The differences between the modeled and observed winds illustrate the pervasiveness and importance of variability in the atmosphere: in the high‐latitude energy and momentum sources, in the solar forcing, and in the waves that originate in the lower atmosphere and penetrate the thermosphere.

Thermospheric nighttime neutral temperature and winds over Fritz Peak Observatory: Observed and calculated solar cycle variation

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer for nearly 17 years between 1968 and 1985. We use data for two specific November periods, (1) 1984, near solar cycle minimum, and (2) 1979, near the peak of solar cycle 21, to illustrate the measured solar cycle variation of nighttime neutral gas temperature and winds over the station. In particular, we present data for November 11, 1979, the day following the day when the solar F10.7 radio flux emission reached its greatest daily value of 367 J (1 J = 1.0×10−22 W m−2 Hz−1). The nighttime measurements, all selected for geomagnetic quiet conditions, show a considerable variation of the thermospheric temperature between solar minimum and solar maximum of nearly 500 K but a relatively minor variation in the thermospheric winds. The recently developed National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) is used to simulate the geomagnetic quiet time variation of global thermospheric properties for the two periods and also to perform a time‐dependent simulation to calculate the thermospheric variation during November 9–12, 1979, when the solar F10.7 flux varied from 314 J, 367 J, 325 J, and 294 J, respectively. The TIE‐GCM histories are used to construct the diurnal variation of thermospheric temperatures and winds as a function of altitude over Fritz Peak Observatory. The aeronomy calculated by the TIE‐GCM is also used to predict the diurnal variation and height of the 630‐nm volume emission rate. The station processor calculates the emission‐weighted height‐integrated Doppler line profiles and Doppler shift profiles as would be observed from a ground station. These profiles are reduced into winds and temperatures in a manner similar to the experimental measurements and can be compared with the actual observations. The results show that the TIE‐GCM calculated Doppler temperature and winds are in reasonable agreement with the observations for the two periods representing solar minimum and extreme maximum conditions, suggesting that the solar flux model and aeronomic processes in the TIE‐GCM can simulate the main thermospheric variations throughout the solar cycle. Furthermore, the time‐dependent calculated variations of Doppler temperature and winds also remain in good agreement with the observations of November 11, 1979, which is the day following the maximum solar EUV perturbation of the Earth's upper atmosphere during the entire solar cycle 21. These results show the need for time‐dependent calculations when geophysical parameters have large changes during the course of the period of simulation. The station processor results indicate that the altitude of the 630‐nm emission varies during the solar cycle by keeping at a near‐constant pressure surface, whose geometrical height changes with the solar cycle.

Rapid recovery of anomalous cosmic ray flux at 1 Au in solar cycle 22

A new observation of low energy cosmic ray particles with the energies from 4 to 120 MeV/n by the HEP instrument onboard the GEOTAIL satellite orbiting at 1 AU shows a remarkable enhancement of anomalous cosmic ray (ACR) N, O and Ne in the period September 1992–December 1993 before approaching to the solar minimum in solar cycle 22. The ACR fluxes obtained by the GEOTAIL observation are in good agreement with the results earlier obtained from the SAMPEX satellite [Mewaldt et al. 1993a,b]. The ACR carbon is also evident though the C enhancement is small compared with those of N, O and Ne. We confirm the new SAMPEX finding that the ACR fluxes have recovered more rapidly in 1992–1993 than in previous solar cycles and find this tendency continues to the end of 1993.

Database of theoretical thermosphere/ionosphere predictions

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model has performed a series of self‐consistent calculations of the thermosphere/ionosphere structure for a variety of geophysical conditions. The model simulates March, June, and December periods for solar cycle minimum and maximum for three levels of geomagnetic activity representing quiet, moderate, and disturbed conditions. The calculations include predictions of N2, O2, O, N(4S), N(²D), NO, N2+, O2+, O+, NO+, and N+, along with the ion, electron, and neutral temperatures and the three components of the neutral winds. The model simulations are available from the authors or through the Coupling, Energetics, and Dynamics of Atmospheric Regions database at NCAR.

Energy deposition and middle atmosphere electrodynamic response to a highly relativistic electron precipitation event

Rocket data have been used to evaluate the characteristics of precipitating relativistic electrons and their effects on the electrodynamic structure of the middle atmosphere. These data were obtained at Poker Flat, Alaska, on May 13 and 14, 1990, during a midday, highly relativistic electron (HRE) precipitation event. Solid state detectors were used to measure the electron fluxes and their energy spectra. An X ray scintillator was included on each flight to measure brems Strahlung X rays produced by energetic electrons impacting on the upper atmosphere. However, these were found to be of negligible importance for this particular event. The energy deposition by the electrons has been determined from the flux measurements and compared with in situ measurements of the atmospheric electrical response. The electrodynamic measurements were obtained by the same rockets and additionally on May 13, with an accompanying rocket. The impacting flux was highly irregular, containing short‐lived bursts of relativistic electrons, mainly with energies below 0.5 MeV and with fluxes most enhanced between pitch angles of 0°–20°. Although the geostationary counterpart of this measured event was considered to be of relatively low intensity and hardness, energy deposition peaked near 75 km with fluxes approaching an ion pair production rate in excess of 100/cm3 s. This exceeds peak fluxes in relativistic electron precipitation (REP) events as observed by us in numerous rocket soundings since 1976. Conductivity measurements from a blunt probe showed that negative electrical conductivities exceeded positive conductivities down to 50 km or lower, consistent with steady ionization by precipitating electrons above 1 MeV. These findings imply that the electrons from the outer radiation zone can modulate the electrical properties of the middle atmosphere to altitudes below 50 km. During the decline and activity minimum of the current solar cycle, we anticipate the occurrence of similar events but with fluxes 1–2 orders of magnitude above that reported here, based on studies of earlier solar cycles (e.g., Baker et al., 1993).

Solar wind velocity and temperature in the outer heliosphere

At the end of 1992, the Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were at heliocentric distances of 56.0, 37.3, and 39.0 AU and heliographic latitudes of 3.3°N, 17.4°N, and 8.6°S, respectively. Pioneer 11 and Voyager 2 are at similar celestial longitudes, while Pioneer 10 is on the opposite side of the Sun. All three spacecraft have working plasma analyzers, so intercomparison of data from these spacecraft provides important information about the global character of the solar wind in the outer heliosphere. The averaged solar wind speed continued to exhibit its well‐known variation with solar cycle: Even at heliocentric distances greater than 50 AU, the average speed is highest during the declining phase of the solar cycle and lowest near solar minimum. There was a strong latitudinal gradient in solar wind speed between 3° and 17°N during the last solar minimum, but this gradient has since disappeared. The solar wind temperature declined with increasing heliocentric distance out to a heliocentric distance of at least 20 AU; this decline appeared to continue at larger heliocentric distances, but temperatures in the outer heliosphere were surprisingly high. While Pioneer 10 and Voyager 2 observed comparable solar wind temperatures, the temperature at Pioneer 11 was significantly higher, which suggests the existence of a large‐scale variation of temperature with heliographic longitude. There was also some suggestion that solar wind temperatures were higher near solar minimum.

A thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (time‐GCM): Equinox solar cycle minimum simulations (30–500 km)

A new simulation model of the mesosphere, thermosphere, and ionosphere with coupled electrodynamics has been developed and used to calculate the global circulation, temperature and compositional structure between 30–500 km for equinox, solar cycle minimum, geomagnetic quiet conditions. The model incorporates all of the features of the NCAR thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) but the lower boundary has been extended downward from 97 to 30 km (10 mb) and it includes the physical and chemical processes appropriate for the mesosphere and upper stratosphere.The first simulation used Rayleigh friction to represent gravity wave drag in the middle atmosphere and although it was able to close the mesospheric jets it severely damped the diurnal tide. Reduced Rayleigh friction allowed the tide to penetrate to thermospheric heights but did not close the jets. A gravity wave parameterization developed by Fritts and Lu (1993) allows both features to exist simultaneously with the structure of tides and mean flow dependent upon the strength of the gravity wave source, The model calculates a changing dynamic structure with the mean flow and diurnal tide dominant in the mesosphere, the in‐situ generated semi‐diurnal tide dominating the lower thermosphere and an in‐situ generated diurnal tide in the upper thermosphere. The results also show considerable interaction between dynamics and composition, especially atomic oxygen between 85 and 120 km.

Solar cycle and seasonal variations in F region electrodynamics at Millstone Hill

Incoherent scatter radar observations of ion drifts taken at Millstone Hill (42.6°N, 288.5°E) during 73 experiments in the period February 1984 to February 1992 are used to construct, for the first time at this station, average quiet‐time E×B drift patterns for both solar cycle maximum and minimum, for the summer, winter, and equinox seasons. The daily variation of V⊥N shows a reversal from northward to southward drifts near noon, and a return to northward drifts in the premidnight hours. The weaker southward drift in the afternoon in summer noted by Wand and Evans (1981) is shown to occur only at sunspot minimum. The daily variation of V⊥E shows daytime eastward drifts and nighttime westward drifts, except in summer when the usual daytime eastward maximum near 1200 LT is suppressed. The daily mean drift is westward for all seasons, and is largest in summer. The daytime eastward drift and nighttime westward drift tend to be stronger at solar maximum than at solar minimum. Average drift patterns are also constructed for equinox for both extremely quiet and geomagnetically disturbed periods. V⊥N is appreciably more northward under extremely quiet than under disturbed conditions in the postmidnight and morning periods. During extremely quiet periods, V⊥E turns slightly eastward in the evening hours, while it is strongly westward for disturbed conditions. This result contrasts with the strong eastward drifts in the evening in summer reported for extremely quiet conditions at Millstone Hill by Gonzales et al. (1978). A strong anticorrelation is seen at Millstone Hill between V⊥N and V∥, as is found at lower latitude stations. The quiet‐time patterns are discussed in terms of the causative E and F region dynamo mechanisms. At Millstone Hill, conjugate point electric fields are also important in winter when the conjugate ionosphere is sunlit for much of the night.