Period Of Low Sunspot Activity Research Articles

A new criterion for determining the time of a solar minimum

We propose a minimum level of the smoothed values for the solar constant during a period of low sunspot activity as a new additional criterion for determining the time of a minimum between solar cycles. An indicator for the time of a minimum between cycles is the time at which a minimum level in the average monthly values of the integral flux of solar radiation smoothed over thirteen months (when the last four values of the flux are greater than the previous minimum point) is achieved. We successfully tested the new criterion to determine the time of the previous minima between cycles 21 and 22, 22 and 23, and 23 and 24.

UV solar irradiance low during recent solar minimum

Solar irradiance, which varies with the 11‐year solar cycle and on longer time scales, can affect temperatures and winds in the atmosphere, influencing Earth's climate. As the Sun currently wakes up from a period of low sunspot activity, researchers want to know how irradiance during the recent solar minimum compares to historical levels. In addition to understanding the total received power, it is important to know how various spectral bands behave, in particular, the ultraviolet, which causes heating and winds in the stratosphere. Lockwood analyzed solar ultraviolet spectral irradiance data from May 2003 to August 2005 from both the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the Upper Atmosphere Research Satellite (UARS) and the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument on the Solar Radiation and Climate Experiment (SORCE) satellite. Using several different methods to intercalibrate the data, he developed a data composite that can be used to determine differences between the recent solar minimum and previous minima. The author found that solar irradiance during the recent sunspot minimum has been especially low. (Journal of Geophysical Research‐Atmospheres, doi:10.1029/2010JD014746, 2011)

GPS based TEC measurements for a period August 2008– December 2009 near the northern crest of Indian equatorial ionospheric anomaly region

In recent years, measurements of total electron content (TEC) have gained importance with increasing demand for the GPS-based navigation applications in trans-ionospheric communications. To study the variation in ionospheric TEC, we used the data obtained from GPS Ionospheric Scintillation and TEC monitoring (GISTM) system which is in operation at SVNIT, Surat, India (21.16°N, 72.78°E) located at the northern crest of equatorial anomaly region. The data collected (for the low sunspot activity period from August 2008–December 2009) were used to study the diurnal, monthly, seasonal semi-annual and annual variations of TEC at Surat. It was observed that the diurnal variation at the region reaches its maximum value between 13:00 and 16:00 IST. The monthly average diurnal variations showed that the TEC maximizes during the equinox months followed by the winter months, and are lowest during the summer months. The ionospheric range delay to TEC for the primary GPS signal is 0.162 m per TECU. The diurnal variation in TEC shows a minimum to maximum variation of about 5 to 50 TECU (in current low sunspot activity periods). These TEC values correspond to range delay variations of about 1 to 9 m at Surat. These variations in the range delay will certainly increase in high sunspot activity periods. Detected TEC variations are also closely related to space weather characterizing quantities such as solar wind and geomagnetic activity indices.

Research Spotlight: Low solar irradiance may not be the primary driver for the Little Ice Age

Total solar irradiance (TSI), essentially a measure of the amount of light the Sun puts out, varies with the 11‐year sunspot cycle and influences Earth's climate, especially when TSI is notably higher or lower than its average values. It had been thought that TSI was especially low during a period known as the Little Ice Age, which began in the late seventeenth century and coincided with a period of unusually low sunspot activity known as the Maunder Minimum. However, Schrijver et al. now suggest that TSI during that period may not have been as low as previously thought. They analyzed direct measurements of solar magnetic activity during the recent 2008–2009 period of low sunspot activity, which they argue was similar to the activity level during the Maunder Minimum. (Geophysical Research Letters, doi:10.1029/2011GL046658, 2011)

Students, Scientists, and Family Commemorate the Life and Diverse Works of Jack Eddy

Eddy Cross‐Disciplinary Symposium on Sun‐Climate Research; Aspen, Colorado, 22–24 October 2010; In 1976, John Allen Eddy published a seminal article (see Science, 192(4245), 1189–1202) revealing a link between the Little Ice Age, which occurred during the sixteenth through nineteenth centuries, and a period of low sunspot activity, which Eddy called the “Maunder Minimum.” This work placed Sun‐climate research on a firm scientific footing. Eddy passed away on 10 June 2009. Following Eddy's passions for education and cross‐disciplinary research, a symposium was held to expose talented college students to the science and politics of Sun‐climate research. Funding from NASA's Living With a Star Targeted Research and Technology program and from the High Altitude Observatory, Advanced Study Program, and Integrated Science Program of the National Center for Atmospheric Research (NCAR) supported keynote speakers and provided scholarships for 30 students (junior year to Ph.D.) from diverse disciplines. Eddy's wife, Barbara, led a session devoted to personal recollections. Spencer Weart (American Institute of Physics) gave an after‐dinner tribute using recordings of Eddy from a 1999 interview.

Spatial distribution of ionization in the equatorial and low‐latitude ionosphere of the Indian sector and its effect on the pierce point altitude for GPS applications during low solar activity periods

The equatorial and low‐latitude ionosphere is characterized by dynamic changes in the spatial distribution of F region electron density. The in situ density data from Indian SROSS C2 satellite was used in conjunction with the international reference ionosphere (IRI) to demonstrate how the median models have limitations to represent the spatial gradients in the equatorial and subtropical latitudes in the Indian sector. The ground‐based ionosonde data was used to derive the bottomside electron density profiles which were matched with IRI to derive the complete F region electron density profile for different times to show the variability in the centroid of ionization mass distribution due to the equatorial plasma dynamics associated with Appleton ionization anomaly. The study brings out that a uniform ionospheric pierce point altitude cannot be assumed for GPS applications in this sector. However, during low sunspot activity periods, the GPS‐derived total electron content (TEC) using 2 × 2 grid point data indicates that spatial gradients in TEC are not too significant in meridian transects and that the assumption of homogeneous distribution is valid for the conversion of slant to vertical TEC even in the equatorial latitudes during solar minimum. Keeping this in view, a qualitative study has been carried out on the possible variability in ionospheric pierce point altitude at representative subtropical latitude in the Indian sector and the results are discussed in light of diurnal and seasonal features of the equatorial ionosphere.

Temporal and spatial variations in TEC using simultaneous measurements from the Indian GPS network of receivers during the low solar activity period of 2004–2005

Abstract. With the recent increase in the satellite-based navigation applications, the ionospheric total electron content (TEC) and the L-band scintillation measurements have gained significant importance. In this paper we present the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, for the first time, using the Indian GPS network of 18 receivers located from the equator to the northern crest of the equatorial ionization anomaly (EIA) region and beyond, covering a geomagnetic latitude range of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005. The diurnal variation in TEC at the EIA region shows its steep increase and reaches its maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad and occurs around 16:00 LT. A short-lived day minimum occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region. Beyond the crest region the day maximum values decrease with the increase in latitude, while the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00 LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the equator and about 5 to 90 TEC units at the EIA crest region, which correspond to range delay variations of about 1 to 8 m at the equator to about 1 to 15 m at the crest region, at the GPS L1 frequency of 1.575 GHz. The day-to-day variability is also significant at all the stations, particularly during the daytime hours, with maximum variations at the EIA crest regions. Further, similar variations are also noticed in the corresponding equatorial electrojet (EEJ) strength, which is known to be one of the major contributors for the observed day-to-day variability in TEC. The seasonal variation in TEC maximizes during the equinox months followed by winter and is minimum during the summer months, a feature similar to that observed in the integrated equatorial electrojet (IEEJ) strength for the corresponding seasons. In the Indian sector, the EIA crest is found to occur in the latitude zone of 15° to 25° N geographic latitudes (5° to 15° N geomagnetic latitudes). The EIA also maximizes during equinoxes followed by winter and is not significant in the summer months in the LSSA period, 2004–2005. These studies also reveal that both the location of the EIA crest and its peak value in TEC are linearly related to the IEEJ strength and increase with the increase in IEEJ.

Morphological and spectral characteristics of L-band and VHF scintillations and their impact on trans-ionospheric communications

Abstract Amplitude scintillations recorded at 1.5 GHz frequency during the high (1998–1999) and low (2004–2005) sunspot activity periods over a low latitude station, Waltair (17.7°N, 83.3°E) revealed that the L-band scintillations mostly occur during the post-sunset to midnight hours peaking around 21:00 hr local time with maximum occurrence during equinoxes, moderate during winter and minimum during the summer months. The occurrence, as well as the intensity of scintillations, is found to be strongly dependant on both the season of the year and the sunspot number. Strong (S4-index >0.45) and fast fading scintillations (fading rates >40 fads/min) observed during the post-sunset hours of equinoxes and winter months manifest as several short duration patches at both VHF (244 MHz) and L-band (1.5 GHz) frequencies and are found to be always associated with the range or total Spread-F on ionograms and bubbles/depletions in the Total Electron Content (TEC) measured from a colocated dual frequency GPS receiver, suggesting that these scintillations are of the Plasma Bubble Induced (PBI) type. On the other hand, relatively weak and slow fading scintillations (fading rates <8 fads/min) observed around the post-midnight hours of the summer months which appear as long-duration patches (>3 hr) at 244 MHz signal (with practically no scintillation activity at the L-band frequencies) are often found to be associated with frequency Spread-F on ionograms with no depletions in TEC. Further, the presence of Fresnel oscillations observed in the spectrum of 244 MHz suggests that the long-duration scintillations observed are due to the presence of a thin layer of irregularities in the bottom side F-region which are generally known as Bottom Side Sinusoidal (BSS) irregularities. Further, the PBI-type scintillations at L-band frequencies are often found to exceed 10 dB power levels (S4 > 0.45) even during the low sunspot activity period of 2004–2005, and cause Loss of Lock in the GPS receivers resulting in a total interruption in the received signals.

Evening enhancements in F-region electron temperature at subtropical latitudes during June solstice in the Indian SROSS C2 RPA data

In-situ measurments of the topside F-region ionospheric electron and ion temperatures are very few over the low and equatorial latitudes during the last two solar cycles, particularly in the Indian sector. The SROSS C2 satellite has provided some valuable data on the thermal structure of the topside ionosphere over the Indian region. This article reports a typical evening enhancement in the topside F-region electron temperatures around 18:00 IST observed in the subtropical latitudes of 15–20°N. These enhancements that are seen during the low sunspot activity periods show a latitudinal difference with an early and sharp peak at higher latitudes (23°N). The observed features are explained on the basis of equatorial plasma dynamics associated with the Appleton ionization anomaly.

North-South Asymmetry of the Polar Solar Wind and Some Characteristics of Solar Magnetic Field

AbstractWe have found correlated variations of the yearly averaged north-south asymmetry in the polar solar wind speed (Δsol) and the ratio of the zonal quadrupolar to the zonal dipolar contribution in the inferred coronal magnetic field during the declining phase of sunspot cycle 21. A physically meaningful association between Δsol and some polar solar magnetic field proxies is also observed during the low sunspot activity periods of the above cycle.

North-south asymmetry of the polar solar wind and some characteristics of solar magnetic field

We have found correlated variations of the yearly averaged north-south asymmetry in the polar solar wind speed (Δsol) and the ratio of the zonal quadrupolar to the zonal dipolar contribution in the inferred coronal magnetic field during the declining phase of sunspot cycle 21. A physically meaningful association between Δsol and some polar solar magnetic field proxies is also observed during the low sunspot activity periods of the above cycle.

A Study of Ionospheric Characteristics at Trivandrum during High and Low Sunspot Activity Periods

Analysis of ionospheric data obtained during low sunspot activity period corresponding approximately to the first half of IQSY at Trivandrum is considered in this paper. The diurnal and seasonal variation of f0F2, hpF2 and fEs are described. The N-h profiles obtained have been utilized to study the diurnal and seasonal variations of electron density at different heights and the subpeak electron content of the ionosphere. The true heights of maximum electron-density are compared with those obtained by assuming a parabolic distribution.The above results are compared with those obtained during high sunspot activity and the similarities and differences pointed out.

Solar Terrestrial Relationships

The identification of moderate geomagnetic activity with various solar regions has been discussed in the light of the long sequences of annual variation (30 to -50 solar rotations) obtained from the analysis of geomagnetic activity of the low sunspot activity periods 1920-24, 1930-34, 1940-44 (no long sequences) and 1950-54 and solar axial hypothesis. On the basis of these identifications it is shown that the geomagnetic activity can broadly be grouped in three heads-(l) Intense geomagnetic storms, (2) disturbed M -storms and (3) quiet M-storms.
 
 Various features of these geomagnetic storms are qualitatively explained from the focussing of solar-ion streams taking account of the local and general solar a It is found that the streams emerging from the solar regions of large magnetic field may get dispersed, while they may come out in the form of concentrated streams from quiet solar regions which do not possess large magnetic fields. Further qualitative explanation for the formation of coronal plumes near the Sunspots and of coronal streamers around the sunspot minima have also been given.

The morphology of lunar semi-diurnal variation in [formula omitted] near solar noon

The variations of the midday critical frequency of the F2-layer, ƒ 0F2 , with the phase of the moon are computed for eighteen stations within about ±40° latitude during a period of low sunspot activity. The results show that the phase of the lunar variation of ƒ 0F2 is determined by the magnetic and not by the geomagnetic or the geographic latitude. The reversal from the equatorial type of variation with a maximum near 04 lunar hour to the higher latitude type with a maximum near 10 lunar hour occurs at about ±11° magnetic latitude. The variation of amplitude with latitude shows a sharp maximum on the magnetic equator, associated with the equatorial electro-jet, and two broad maxima at about ±20° magnetic latitude. The latter are closely associated with the well-known maxima in the variation of noon ƒ 0F2 with latitude.

Temperature of the F 2 region of the Ionosphere over Kodaikanal

In this paper noon temperature of the F2 region of the ionosphere over Kodaikanal has been computed by the scale height method for the months of June and December of the years 1953 and 1954, the period of low sunspot activity of the present solar cycle. The mean temperature at a height of 400 km is found to be about 15000K. It is also observed that winter temperatures are higher than slimmer temperatures. This summer to winter warming up may be due to the winds in the ionosphere.

The 27-day variation in cosmic ray intensity

The following facts emerge from the study: 1) In general there is a negative correlation betweenI, the cosmic ray intensity, andK bp the planetary index, although there are exceptions. 2) There is a positive correlation between the relative decrease in intensity, —I/It and the planetary index,K bp . There are exceptions during the low sunspot activity period of 1949 ÷ 54. 3) TheK bp minimum (or maximum) is observed to precede the corresponding cosmic ray one in some years and follow in other years. But it appears that the latter case is generally found during the rising part of the solar cycle and possibly during the early fall. 4) There is clear evidence that the amplitude of the 27 day variation in cosmic ray intensity follows the geomagnetic disturbance better than the solar activity. Both of them exhibit similar departure from solar activity during 1946 ÷ 52. 5) The time of maximum of the diurnal variation of cosmic ray intensity on disturbed days follows the geomagnetic disturbance better than the solar activity. The semidiurnal time of maximum on disturbed days appears to follow the solar activity in a rough measure.

The Distribution of Radio Brightness over the Solar Disk at a Wavelength of 21 Centimetres. III. The Quiet Sun ? Two-Dimensional Observations

A distribution of solar radio brightness at a wavelength of 21 cm has been derived from observations made during the period of low sunspot activity from 1952 to 1954. The observations were made using two multiple interferometers arranged at right angles; this enabled the solar disk to be scanned in many different directions. The derived one-dimensional profiles of the quiet Sun for these various scanning directions were combined and a Fourier method adopted to derive a two-dimensional brightness distribution. The distribution shows marked limb-brightening in the equatorial zones but none in the polar regions. The contours of brightness are in general conformity with those expected from a solar atmosphere having a coronal electron density distribution of the kind proposed by van de Hulst for the period of minimum sunspot activity.

Continuous electron density measurements up to 200 km

Measurements in the ionosphere using the Seddon propagation technique were conducted with the Viking 10 rocket, launched at 10:00 a.m. MST on May 7, 1954, at the White Sands Proving Ground, New Mexico. This rocket reached a record altitude of 219 km, and provided very good data for both upward and downward trajectories. Ordinary and extraordinary indices of refraction at 7.754 Mc were measured in the region surrounding the rocket. From these indices, it was possible to determine accurately the electron distribution from 84 to 200 km. By using the P′-f records taken during the rocket flight, the electron density curve was extrapolated up to the maximum of the F2-region (2.9×105 el/cc at 288 km). The measurements showed a rapid increase in density from 1×104 el/cc at 91 km to 1×105 el/cc at 101 km. This was followed by additional but less rapid increases up to the peak of the F1-region (2.1×105 el/cc at 170 km). Densities in the 170- to 200-km region were, in general, five to ten per cent lower than the F1 maximum density. Although obtained during a period of low sunspot activity, these latest results agree remarkably well with previous results reported by Seddon and Jackson for periods near the sunspot maximum. In all cases, the daytime ionosphere was seen to remain dense throughout the E-region and up to the F-region, with only occasional minor peaks in density. In all cases, the distribution obtained agreed very closely with the P′-f records. Sporadic echoes are apparently due to partial reflections from high gradient regions.