Periods Of Low Magnetic Activity Research Articles

Aurora Records Reveal Shortened Solar Cycle During Maunder Minimum

Fastidious night sky observations from Korean historical texts provide a novel source of evidence for an altered solar cycle during periods of low magnetic activity.

Accuracy Estimation of the Modern Core Magnetic Field Models Using DMA-Methods for Recognition of the Decreased Geomagnetic Activity in Magnetic Observatory Data

A new approach to identify the signals of the Earth’s main magnetic field (core field) based on the magnetic observatory data processing is suggested. The algorithms implemented in the approach are based on the Discrete mathematical analysis (DMA). The developed method is used to analyze the data from 49 midand low-latitude observatories of the INTERMAGNET network collected in 2011–2015. The results are compared with the classical method for determining the periods of low magnetic activity of external origin which is adopted by the International Association for Geomagnetism and Aeronomy (IAGA). The advantages of the suggested new approach are demonstrated. Based on the data records for the selected time intervals, the time series of the core field components and their secular variations are obtained for each observatory. These data are compared to the values predicted by the most accurate core field models: SIFM, CHAOS-6, and EMM-2015. The accuracy of the models is estimated using a set of statistical parameters: Pearson’s coefficient of linear correlation, Spearman’s and Kendall’s coefficients of rank correlation, the mean and median values over the data sets, and the mean difference between the data obtained by the suggested method from observatory measurements and the model predictions.

Polar magnetic fields and coronal holes during the recent solar minima

AbstractThe slow decline of solar Cycle 23 combined with the slow rise of Cycle 24 resulted in a very long period of low magnetic activity during the years 2007–2009 with sunspot number reaching the lowest level since 1913. This long solar minimum was characterized by weak polar magnetic fields, smaller polar coronal holes, and a relatively complex coronal morphology with multiple streamers extending to mid latitudes. At the same time, low latitude coronal holes remained present on the Sun until the end of 2008 modulating the solar wind at the Earth in co-rotating, fast solar wind streams. This magnetic configuration was remarkably different from the one observed during the previous two solar minima when coronal streamers were confined near the equator and the fast solar wind was mainly originating from the large coronal holes around the Sun's poles. This paper presents the evolution of the polar magnetic fields and coronal holes during the past minimum, compare it with the previous minima, and discuss the implications for the solar wind near the Earth. It also considers the minimum of Cycle 23 in an historical perspective and, in particular, compares it to the long minima at the turn of the 19th century.

Distribution characteristics of topside ionospheric density irregularities: Equatorial versus midlatitude regions

The global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between ±35° geographic latitudes has been studied with the ROCSAT data during moderate to high solar activity years of 1999 to 2004. The result indicates that the global occurrence distribution of the intermediate‐scale (0.1 to 50 km) density irregularities can be grouped into two different populations, one in the equatorial region and the other in the middle‐to‐subauroral latitude region. The global seasonal/longitudinal (s/l) distribution of equatorial irregularities in the current report reproduces the result of McClure et al. (1998) obtained with the AE‐E observations of the mesoscale (50 to 1000 km) plasma bubble structures during high solar activity years of 1978 to 1980, two solar cycles ago. This implies that the density irregularities of different scales from multistage cascading process of the large‐scale (>1000 km) gravitational Rayleigh‐Taylor instability have manifested in same global s/l distribution pattern. Furthermore, global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years. In addition, the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about ±30°. A different kind of midlatitude irregularity distribution starts to fill in from this dip latitude. In other words, the equatorial density irregularity inside a depleted flux tube can only rise, on statistical average, to an apex height of ∼2000 km. Different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes. The occurrence frequency of equatorial density irregularities increases with solar flux intensity; whereas the midlatitude density irregularity is more likely to occur during low solar activity period. The equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times. On the other hand, the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity. The local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight. Such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate‐scale density irregularities.

Diffusion of 155 to 430 keV protons in the Earth's radiation belt

Pitch angle distributions and energy spectra of 155–430 keV trapped protons between L=4 and L=5.5 are examined to study pitch angle and radial diffusion rates during a period of low magnetic activity. To improve the sensitivity to pitch angle diffusion, the measurements were made near the loss cone where angular gradients in the distribution function are large. In this region the phase space density is such that both radial diffusion and pitch angle diffusion lead to flux increases while charge exchange leads to proton loss. However, the conventional prescriptions for the diffusion coefficients predict too much diffusion at L=4, implying growth in the proton fluxes while observations show loss. At L=5, theory and observation both show slight growth, and at low pitch angles diffusion in pitch angle is necessary to compensate for charge‐exchange losses. Although Dst was low during the 13 days preceding the measurements, the proton belt had not reached a steady state with diffusion sources matching charge‐exchange losses.

A Picture of Solar Minimum and the Onset of Solar Cycle 23. I. Global Magnetic Field Evolution

NSO/Kitt Peak synoptic charts of magnetic flux in the period from 1996 to 1998 are analyzed together with time series for the 10.7 cm radio flux, sunspot number, and Mg II chromospheric index to determine the origin of the two times of minimum activity in 1996 and to study their relationships in the ascending phase of solar cycle 23. The solar activity minima in February-April and September-November of 1996 are found to correspond to periods of low magnetic activity in the southern and northern solar hemispheres, respectively. The new solar cycle becomes dominant in early 1997, but it is only in the summer of 1997 that a significant increase can be detected in the magnetic fields observations as well as in irradiance data, and by the end of 1998, the activity level has increased to a value comparable to the one observed in 1993. Using the magnetic synoptic charts, we determine the number of persistent longitude bands of active nests during this rising phase of solar cycle 23. We find an increase in the number of active nests from zero in 1996 to three by 1998. We speculate that these persistent bands of flux emergence correspond to a pattern of low-order modes of instability of the type found in the theoretical work of Gilman, Fox, and Dikpati on joint instability of latitudinal differential rotation and toroidal magnetic fields at the base of the convection zone. We argue that the observed increase in the number of active nests is consistent with the increase in the longitudinal wavenumber of magnetic instabilities in a concentrated toroidal field in the tachocline discussed in 1999 by Gilman and Dikpati.

SABRE observations of Pi2 pulsations: case studies

Abstract. The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.

SABRE observations of Pi2 pulsations: case studies

<strong class="journal-contentHeaderColor">Abstract.</strong> The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.

Nocturnal high-latitude E-region in winter during extremely quiet conditions

The quiet night-time E-region at high latitudes has been studied using the EISCAT UHF radar. Data from three subsequent nights during a long period of low magnetic activity are shown and typical features of electron density are described. The background electron density is observed to be 5·10 9 m −3 or smaller. Two types of enhancements above this level are observed ; one is due to charged particle precipitation associated with the F-region trough and the other is composed of sporadic- E layers due to waves in the neutral atmosphere. The sporadic- E is observed to exist almost continuously and to exhibit a regular diurnal behaviour. In addition to the typical afternoon and morning sequential layers, a third major descending layer is formed at night after the passage of the F-region trough The afternoon layer disappears simultaneously with the enhancement of the northward trough-associated electric field and the night-time layer appears at high altitudes after the field has again been reduced to a small value. It is suggested that metal ions from low altitudes are swept by the electric field to the upper E-region where they are again compressed to the night-time layer. A set of steeply descending weaker layers, merging to the main night-time layer are also observed. These layers are most probably caused by atmospheric gravity waves. Theoretical profiles for molecular ions indicate that the strongest layers are necessarily composed of metal ions but, during times when the layers are at their weakest, they may be mainly composed of molecular ions.

Permanent flare activity in the magnetosphere during periods of low magnetic activity in the auroral zone

Auroral, magnetic variation and pulsation data from the dense network in the nearmidnight portion of the auroral zone are used together with the measurements of suprathermal particles and electromagnetic fields by the IMP-8 and ISEE-1 spacecraft within the plasma sheet to study the characteristics of activity during two magnetically quiet periods on 3 March 1976 and 23 March 1979. Contrary to existing beliefs, we found clear signatures of numerous (5–10 events per hour) transient events, characterized by plasma flows, energetic particle bursts and E− B field variations. A close association of these events in the plasma sheet with the local auroral flares (LAFs) in the conjugate sector of the auroral zone is established for many events. We conclude that LAF (local auroral arc activation with associated Pi pulsations but extremely weak magnetic bays) have the same plasma sheet manifestations (apparently, the same physics) as the individual substorm intensifications during strong substorm expansion events, which differ from the studied quiet periods mainly by the strength and number of these intensifications. These transient phenomena seem to play an important role in the energetics of the quiet time magnetotail.

Observations of electric fields near the plasmapause at midnight

We report here static electric field measurements in the equatorial plane from the 100‐m double floating probe experiment carried on the SCATHA (P‐78‐2) satellite. Eclipse measurements on auroral L shells are studied to better understand the change in magnetospheric convection at the plasmapause. The use of eclipse data allows the electric fields to be measured directly without contamination from the spacecraft photoelectron sheath; and it allows the thermal ion population in the outer plasmasphere to be observed, identifying the satellite location with respect to the plasmapause. (Such measurements are limited to near local midnight.) We find that in the corotating frame there is generally an electric field component in the antisolar direction during periods of low magnetic activity which begins at the edge of the plasmasphere and builds in amplitude as the satellite enters the plasma sheet. The amplitude of this electric field varies from 0.05 mV/m to 0.2 mV/m with increasing magnetic activity; the electric field switches to radially inward during periods of high activity with magnitudes up to 2 mV/m. The amplitude of the observed electric field increases with the amount of contribution from east‐west currents in the earth's plasma sheet‐magnetotail current system. Since the electric fields observed on these eclipse passes are radially outward in the midnight meridian during periods of low activity and since they are observed on a near‐synchronous satellite which generally corotates with the earth, they are not included in most modeling studies of the formation of the plasmasphere.

ISEE 1 and 2 observations of Birkeland currents in the Earth's inner magnetosphere

Data from the ISEE 1 and 2 magnetometers were examined for evidence of Birkeland currents in the earth's inner magnetosphere, from 2.4‐ to 7‐RE radial distance. This survey reports on the two spacecraft measurements of the properties of the Birkeland currents. Both line and sheetlike currents were observed. Large‐scale Birkeland current signatures were observed throughout the 18 hours of local time in which the data coverage was sufficient. In local time this coverage was from local noon through dusk to local midnight to the dawn meridian. In approximately 27% of the spacecraft's outbound passes, large‐scale current structures were identifiable as parts of the region 1 or region 2 current systems. In approximately 19% of the outbound passes, no distinguishable region 1 or region 2 Birkeland currents were observed. These cases occurred during periods of low magnetic activity. In the remaining 54%, both multiple current structures and ambiguous magnetic signatures were observed. The multiple structures were generally of smaller scale than the structures identified as the region 1 and 2 current structures, and the ambiguous cases had no clear sources or causes. For these cases the spatial and temporal variations were not easily separated. Two passes through the dayside magnetosphere showed Birkeland current layers at the interface between the outer plasma sheet convection boundary and a plasma mantlelike boundary region, located inside the postnoon magnetopause. These two cases provided clear examples of region 1 Birkeland current flow in the dayside magnetosphere.

Mass composition of substorm‐related energetic ion dispersion events

The Lockheed ion mass spectrometer flown onboard the SCATHA (P78‐2) spacecraft is used to study the mass composition of two ion dispersion events. The energy‐dispersed ions are observed over the full energy range of the instrument (0.1–32 keV/q) in the noon‐dusk local time sector. On one of the days, March 22 (day 81) 1979, the dispersing ions are first observed following an isolated substorm. A long period of low magnetic activity is present prior to the substorm on this day, and a decrease in Dst is observed following the first observation of the dispersing ions. On the second day studied, June 7 (day 158), 1979, the correlation between ground magnetic activity and initial observation of dispersing ions is not so clear, since the dispersion follows a period of high magnetic activity. Both dispersion events follow a marked reduction in the preexisting near‐geosynchronous plasma near the noon sector. The mass composition shows that although there are similarities in the dispersion for both protons and oxygen, there are also distinct differences. Both species show a definite dispersion ridge, but the protons also have additional fluxes at energies greater than the dispersion ridge energy. It is proposed that the composition changes are attributed to localized injection of ionospheric plasma in the dusk‐midnight sector, with proton‐rich plasma sheet ions convecting past the spacecraft from larger radial distances. By using the ability to scan in pitch angle, it is noted that there are both pitch angle and mass dependences to the arrival times of the dispersing ions. Protons at 30° pitch angles arrive at the spacecraft before oxygen ions at the same pitch angle.

Quiet‐time electron precipitation at L = 4 in the South Atlantic Anomaly

On January 12, 1978, at 1000 UT, a Superarcas sounding rocket was launched from Siple Station, Antarctica, (L = 4.1, 76°S, 84°W) during a prolonged geomagnetically quiet period (Kp = 1+) with very low VLF activity. The parachuted payload observed a moderately intense charged particle flux that smoothly decreased as the payload descended through the atmosphere. These data have been carefully extrapolated to zero depth in the atmosphere for comparison with both low‐altitude satellite measurements and theoretical predictions of the precipitating flux in the region of the South Atlantic anomaly. The integrated flux was found to agree reasonably well with both the satellite measurements and prediction. The calculated spectrum was found to be much softer than typical trapped electron spectra reported from satellite measurements after extended periods of low magnetic activity. This result is consistent with experimental observations that the lifetime of outer belt particles increases with energy.

Transmission of large-scale TIDs in the ionospheric F2-region

Large-scale travelling ionospheric disturbances (1.s. TIDs) have been investigated in order to derive the horizontal velocity dispersion by using f 0 F2 data from four ionospheric observatories in Japan. It was found that the horizontal phase trace velocity lies between 300 and 1000ms −1 with periods in the range 50 to 150 min. There is evidence that the derived velocity generally increases with increase of wave period. This is consistent with the dispersion predicted by the theory of the internal gravity waves. The azimuthal angles are distributed in ±35° sectors centered around 197° (measured clockwise from north), indicating that 1.s. TIDs may be obtainable when they are excited along the auroral zone of the same sector in longitude as that of the observatories. The average propagation direction shifts by 17° from south towards west. This clockwise shift is consistent with the rotation caused by the Coriolis effect. This means that the Coriolis effect cannot be ignored for the wave propagation of 1.s. TIDs. In addition to the positive correlation between TID speed and geomagnetic activity, the direction of wave propagation is found to be correlated with polar magnetic activity. The propagation direction is mostly southward during the period of large polar magnetic disturbances, while during the period of low magnetic activity the direction scatters considerably.

Adiabatic acceleration induced by convection in the plasma sheet

A study of the average energy and temperature of ions precipitated in the nocturnal auroral zone during periods of low magnetic activity is presented. On the basis of this study, carried out using the Aureole 1 satellite, it can be concluded that the characteristics of ions at auroral altitudes well reflect their convection in the distant plasma sheet and the betatron and Fermi type acceleration accompanying it.

Characteristics of polar cap Sun-aligned arcs

Observations of polar cap sun-aligned arcs obtained with the auroral scanning photometer on Isis 2 for the period 1971 to 1975 are examined. A 2∶1 asymmetry was found in the occurrence frequency between the morning and evening sectors of the polar cap. Sun-aligned arcs were observed on only 0.6% of polar cap passes and occurred most frequently during periods of low magnetic activity (Kp and AE). Moreover, for all cases observed during times for which interplanetary magnetic field data were available, the field was directed northward. Although the intensity along any single arc varied considerably, it was found that the 5577 Å/3914 Å intensity ratio remained constant. Examination of particle data and the observed intensity ratios indicate that the arcs are excited by low-energy (⩽1 keV) electron fluxes.

Comparison of neutral temperatures inferred from instruments on the AE‐C satellite

Four determinations of neutral thermospheric temperature from data taken with instruments aboard the AE‐C spacecraft have been compared during the circular orbit phase in late December 1974 and January 1975 when the satellite altitude was near 260 km. These temperatures are found to be in overall agreement to within about 10% during periods of low magnetic activity (Ap &lt; 12). The methods are based on: (1) analysis of the satellite spin modulation of N2 density; (2) analysis of the ion temperature; (3) and (4) the calculation of the temperature needed to match measured N2 and Ar densities assuming fixed boundary conditions at 120 km. Systematic differences are observed such that the ‘argon temperature’ increases the most from winter to summer and ion temperature the least.

Ionospheric scintillations at 136 MHz from a synchronous satellite

Scintillation observations at 136MHz were made using the Early Bird synchronous satellite during the summer of 1965 from two middle latitude sites. The diurnal pattern of the scintillations had the following characteristics: 1. a. Two maxima were found, one near midday, the other near midnight. 2. b. The patch of irregularities which produced the midday scintillation moved through the antenna beam in a few minutes to 14 min. 3. c. The midday scintillations had a high probability of occurrence when ƒ 0E 8 ≥ 5 MHz . 4. d. The scintillations observed from 1900 to 0300 local time were of long duration (greater than 30 min) 5. e. The start and stop time of night scintillations were well correlated at Greenbank, West Virginia and Sagamore Hill, Massachusetts, a separation of subionospheric latitude of approximately 4 degrees. 6. f. 48 per cent of the nighttime scintillations were accompanied by spread F; 25 per cent of the night hours showed spread F. 7. g. The period of the scintillation (peak to peak time of the individual fluctuation) ranged predominantly from 5 sec to 1 min. 8. The picture that evolves is that summer daytime scintillations during periods of low magnetic activity are probably associated with sporadic- E patches of 300–600 km with high critical frequency sporadic- E. Nighttime scintillation is correlated with spread F, geomagnetieally controlled and occurs over a large latitude range in the same time zone.

Frequency dependence of radio star scintillations

The amplitude scintillations of the radio flux from Cygnus and Cassiopeia have been studied at discrete frequencies in the VHF-UHF range (30, 63, 113, 228, and 400 Mhz) using a 150-foot and an 84-foot diameter steerable paraboloids, both independently and in an interferometer mode. Data at various combinations of these frequencies have been collected over a period of 3½ years and analyzed to establish the frequency dependence of ionospheric scintillations. For weak scintillations (the higher frequencies during periods of low magnetic activity) the scintillation index is found to be inversely proportional to (frequency)². For lower frequencies and high levels of ionospheric activity, the dependence approaches 1/ƒ. These results agree with predictions based upon a simplified theory of weak scattering in the medium and assuming various combinations of irregularity size and height of the disturbing layer. Measurements made with the two antennas in an interferometer configuration established that the diffraction field was truly random as assumed. Correlation coefficients were obtained for discrete sets of data and a distinction made between weak and strong scattering cases. Irregularity sizes of about 1–2 km were found to be consistent with the bulk of the observations, on the basis of weak scattering theory.