Precipitation of > 115 kev protons in the evening and forenoon sectors in relation to the magnetic activity

Data from a low altitude polar orbiting satellite, on auroral protons >115 keV in the evening and forenoon sectors, are presented.In the forenoon sector there is a weak but fairly steady precipitation at Λ ≈ 75° during quiet conditions. This precipitation is situated at higher invariant latitudes at local noon than at local dawn and can probably be ascribed to the high energy tail of the polar cleft protons. During moderately disturbed conditions, especially during the recovery phase of geomagnetic storms, there are some seemingly more “impulsive” precipitation events at Λ ≈ 65°. During very disturbed conditions these two precipitation zones in the forenoon sector seem to merge.In the evening sector a rather sharp equatorward boundary of the main precipitation, at Λ ≈ 69° during quiet conditions, varies fairly smoothly from pass to pass. South of this boundary, at invariant latitudes around 62°, there is a steady weak drizzle from the radiation belt. Due to a longitudinal effect this drizzle, as recorded by the satellite, shows a diurnal variation.The equatorward boundaries of the main precipitation at both local times move equatorward with increasing ring current strength. When D_st gets less than about — 100nT, the poleward boundaries are found to move equatorward too. From an attempt to reveal some of the substorm-dependent changes of the precipitation it is found that an equatorward shift of the precipitation areas takes place during, or just prior to, the substorm expansive phase, accompanied by a large intensity increase in the evening sector, whereas the recovery phase is linked with a poleward expansion of the precipitation at both local times.     

A uniform belt of diffuse auroral emission seen by the ISIS-2 scanning photometer

One of the most striking and persistent features in high latitude regions as seen by the ISIS-2 scanning auroral photometer is a fairly uniform belt of diffuse auroral emission extending along the auroral oval. Indications are that this region follows, contributes to, and may in a sense actually define the auroral oval during quiet times.The diffuse belt is sharply defined at its equatorward edge, which is located at an invariant latitude of about 65° in the midnight sector during relatively low magnetic activity (K_p = 1?3). The poleward edge of the region is not as sharply defined but is typically at about 68°. Discrete auroras (arcs and bands) are located, in general, near the poleward boundary of the diffuse aurora. The position of the belt appears to be relatively unaffected by the occurrence of individual substorms, even when discrete forms have moved well poleward. Representative intensities at 5577 Å are 1–2 kR (corrected for albedo) at quiet times and may reach 5 kR during an auroral substorm.It appears that the mantle aurora and proton aurora constitute this diffuse aurora in the midnight sector. Precipitating protons and electrons both contribute to the emissions in this region.     

Measurements of the E region neutral wind field

The neutral E-region wind field was measured at Calgary, Canada (51°N, 114°W) during 75 nights in 1982. Observations of the Doppler shift of the 5577-Å emission line of atomic oxygen using a Fabry-Perot interferometer were converted to horizontal wind vectors. From the analysis of the data, four categories of wind characteristics were identified. In order of increasing magnetic activity these categories are (a) wind field mostly variable in space and time, (b) predominantly equatorward flow throughout the night, (c) predominantly poleward flow throughout the night and (d) north-westward flow before midnight and southward after midnight. The wind magnitude was also variable and on some disturbed days exceeded 200 m s^?1.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.     

The auroral oval during the substorm development

The direction of motion of the auroral forms in several sectors of the auroral oval during substorms is studied. The creation phase is characterized by the equatorward displacement of the luminous region in evening (15–21 LT) and in day (09–15 LT) hours, while individual forms in the luminous region drift mainly poleward with a mean velocity of 230 m/sec in day hours and equatorward with the mean velocity of 230 m/sec in evening hours. The equatorial shift of the luminous region correlates well with the B_Z-component of the interplanetary magnetic field. The onset of the displacement coincides with the southward B_Z-rotation and is accompanied by auroral intensity increase for about 10–20 min.During the expansive and recovery phases the day auroras drift poleward with mean velocities of 330 and 300 m/sec, respectively. In the evening sector the individual auroral forms drift both poleward and equatorward during the expansive phase and drift mainly towards the pole during the recovery phase with a mean velocity of 200 m/sec. In the morning sector characteristics of the motion of the individual auroral forms are more complicated than in the other sectors. The well defined shifts of the luminous region are not discovered. The possible relation between the motions of individual auroral forms with the magnetosphere convection is discussed.     

Evidence for a poleward meridional flow on the sun

We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find:

1. The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field.
2. Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s^-1.
3. The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward.
4. The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.

     

Three-dimensional current system in different phases of a substorm

It is assumed that the three-dimensional current system of a substorm passes three successive stages. (1) When a dawn-to-dusk magnetospheric electric field appears, a current system with field-aligned currents at the poleward boundary of the auroral zone arises. An equivalent ionospheric current system calculated, taking into account a day-night asymmetry of ionospheric conductivity, looks like the well-known DP-2 system including an eastward low-latitude current and a greater magnitude of the dusk vortex in comparison with the dawn one. (2) An electric drift of plasma towards the Earth leads to the appearance of a westward partial ring current increasing in time. This current is closed by field-aligned currents at the equatorward boundary of the auroral zone. The calculated equivalent current system is similar to the well-known one of the precursory phase. (3) An increase of the auroral ionospheric conductivity during the expansive phase produces an increase of all currents and a turning of field-aligned currents at the equatorward boundary of the auroral zone relative to those at the poleward one. The calculated equivalent current system is similar to the DP-1 system.     

Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: Influence of the westward travelling surge

This paper expands the earlier results of Rostoker and Samson (1981), who noted that there are two latitudinal areas of Pi 2 localization near the high latitude, substorm enhanced electrojets. The detailed study presented here outlines the morphology of the polarizations of the Pi 2's in and near the westward travelling surge. There are two latitudinal areas of Pi 2 localization. A poleward Pi 2 predominates within the surge and to the East, whereas an equatorward Pi 2 predominates equatorward and West of the surge. These Pi 2 localizations appear to correlate with the substorm enhanced westward and eastward electrojets respectively. However, the maximum in the Pi 2 power does not always coincide with the center of the electrojet. The poleward Pi 2 has largest amplitudes to the East of the head of the westward travelling surge. This Pi 2 shows a latitudinal polarization reversal from clockwise on the equatorside (viewed down on H-D plane) to counterclockwise on the poleside of a latitudinal demarcation line, which occurs just poleward of the initial breakup. This demarcation line is usually equatorward of the most poleward expansion of the surge. To the West of the surge front, where the equatorward Pi 2 predominates, there is again a latitudinal polarization reversal but in this case the polarization is counterclockwise equatorward and clockwise poleward of the demarcation line. This demarcation is equatorward of that for the poleward Pi 2, and appears to lie at the latitude of the initial breakup. Consequently, the westward travelling surge appears to mark the longitudinal transition from equatorward to poleward Pi 2. The elliptical polarization of the Pi 2's is most likely caused by azimuthai (longitudinal) expansion of the field-aligned currents in the surge, in association with reflection of the field-aligned current pulses from northern and southern high latitude ionospheres.     

Solar-Cycle Variation of Subsurface Zonal Flow

We study the solar-cycle variation of the zonal flow in the near-surface layers of the solar convection zone from the surface to a depth of 16 Mm covering the period from mid-2001 to mid-2013 or from the maximum of Cycle 23 through the rising phase of Cycle 24. We have analyzed Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) Dopplergrams with a ring-diagram analysis. The zonal flow varies with the solar cycle showing bands of faster-than-average flows equatorward of the mean latitude of activity and slower-than-average flows on the poleward side. The fast band of the zonal flow and the magnetic activity appear first in the northern hemisphere during the beginning of Cycle 24. The bands of fast zonal flow appear at mid-latitudes about three years in the southern and four years in the northern hemisphere before magnetic activity of Cycle 24 is present. This implies that the flow pattern is a direct precursor of magnetic activity. The solar-cycle variation of the zonal flow also has a poleward branch, which is visible as bands of faster-than-average zonal flow near 50° latitude. This band appears first in the southern hemisphere during the rising phase of the Cycle 24 and migrates slowly poleward. These results are in good agreement with corresponding results from global helioseismology.     

The old calcium plages differential rotation latitudinal profile: Its gross and fine structure evolution with the solar cycle

This work is a study of the rotational properties of the solar calcium plages, during the time interval 1967–1977; only plages older than 4 days have been the object of this research. We have looked systematically for any significant change occurring during the course of the solar cycle, and any kind of ‘anomaly’ or fine structure in the differential rotation latitudinal profile. We find that such a profile undergoes a cyclic transformation, making it assume the highest steepness at the solar maximum; a sudden flattening then occurs in the first years of declining activity; the last years of the cycle, as the first years of the next one, are characterized by intermediate steepness values. Moreover, we find that, in spite of the general belief that the angular rotation rate is continuously decreasing with increasing heliographic latitude, at least two inversions do exist of such an overall tendency:

1. A narrow, minimal angular-rotation-rate strip lies very close to the equatorward margin of the plage production band; this feature shifts continuously, in a wave-like manner, throughout the solar cycle, from 15/18° to 3/6° latitude.
2. A narrow, maximal angular-rotation-rate strip has been observed lying in the neighbourhood of the poleward margin of the activity band; a process of continuous transformation of the rotation rate profile is always active, in a narrow latitude strip on the equatorward side of such a feature, generating new features of the same kind, which replace the older ones, that disappeared due to the equatorward shift of the plage zone. All that simulates an equatorward shift of the observed ‘anomalies’; we observed them until the minimum activity epoch (1976), at 15/18° latitude. Some relations of these features with both torsionai waves (Howard and LaBonte, 1980) and magnetic activity are briefly discussed.

     

Studies of solar magnetic fields

Magnetic flux data from the Mount Wilson magnetograph are examined over the interval 1967–1973. The total flux in the north is greater than that in the south by about 7% over this interval, reflecting a higher level of activity in the northern hemisphere. Close to 95% of the total flux is confined to latitudes equatorward of 40°, which means that close to 95% of the flux cancels with flux of opposite polarity before it can migrate poleward of 40°. It is pointed out that a consequence of this flux distribution is that ephemeral regions must make a negligible contribution to the long-term largescale magnetic flux distribution. A broad peak in the total flux may be seen centered about one year after activity maximum in the north below 40°. In the south there is a very sharp increase in flux about the same time. In the north, several poleward migrations of flux may be seen. Two of these may correspond with the two poleward prominence migrations seen by Waldmeier. In both the north and the south there is a poleward migration of negative flux about the time of activity maximum. Poleward flux drift rates are about 20 m s^?1.     

Meridional motions of magnetic features in the solar photosphere

We cross-correlate pairs of Mt. Wilson magnetograms spaced at intervals of 24–38 days to investigate the meridional motions of small magnetic features in the photosphere. Our study spans the 26-yr period July 1967–August 1993, and the correlations determine longitude averages of these motions, as functions of latitude and time. The time-average of our results over the entire 26-yr period is, as expected, antisymmetric about the equator. It is poleward between 10° and 60°, with a maximum rate of 13 m s^–1, but for latitudes below ±10° it is markedly equatorward, and it is weakly equatorward for latitudes above 60°. A running 1-yr average shows that this complex latitude dependence of the long-term time average comes from a pattern of motions that changes dramatically during the course of the activity cycle. At low latitudes the motion is equatorward during the active phase of the cycle. It tends to increase as the zones of activity move toward the equator, but it reverses briefly to become poleward at solar minimum. On the poleward sides of the activity zones the motion is most strongly poleward when the activity is greatest. At high latitudes, where the results are more uncertain, the motion seems to be equatorward except around the times of polar field reversal. The difference-from-average meridional motions pattern is remarkably similar to the pattern of the magnetic rotation torsional oscillations. The correspondence is such that the zones in which the difference-from-average motion is poleward are the zones where the magnetic rotation is slower than average, and the zones in which it is equatorward are the zones where the rotation is faster.Our results suggest the following characterization: there is a constant and generally prevailing motion which is perhaps everywhere poleward and varies smoothly with latitude. On this is superimposed a cycle-dependent pattern of similar amplitude in which the meridional motions of the small magnetic features are directed away from regions of magnetic flux concentration. This is suggestive of simple diffusion, and of the models of Leighton (1964) and Sheeley, Nash, and Wang (1987). The correspondence between the meridional motions pattern and the torsional oscillations pattern in the magnetic rotation suggests that the latter may be an artifact of the combination of meridional motion and differential rotation.     

Nearly simultaneous observations of the conjugate polar cusp regions

The first simultaneous (within 6 min) observations of the low altitude polar cusp regions in the conjugate hemispheres are reported here based on two events detected by the DMSP-F2 and F4 satellites within the same geomagnetic local time sector. It is found that the electron spectra in the cusp are identical in the opposing hemispheres. In one case the observed latitudinal location and extent of the cusps are the same at the two hemispheres. However, in the other case the location of the equatorward boundary of the cusp regions differs by about 2° with drastically different spatial features. It is also found that in one of the events the plasma sheet electron precipitation regions overlap with the cusp regions at lower latitude in both hemispheres. The poleward boundary of these overlapping regions is located at the same latitude on either hemisphere, suggesting that this is the latitude of the last closed field line and that the cusp electrons are present on both closed and open magnetic field lines.     

Cyclical behavior of solar filaments

The Carte Synoptique catalogue of solar filaments from 1919 March to 1957 July, corresponding to complete cycles 16‐18, is utilized to show the latitudinal migrations of solar filaments at low (≤50°) and high (>50°) latitudes and the latitudinal distributions of solar filaments for all solar filaments, solar filaments whose maximum lengths during solar disk passage are less than or equal to 70° and solar filaments whose maximum lengths during solar disk passage are larger than 70°. The results show the following. (1) The latitudinal migrations of all low‐latitude solar filaments and low‐latitude solar filaments whose maximum lengths during solar disk passage are less than or equal to 70° follow the Spörer sunspot law. However, the latitudinal migration of low‐latitude solar filaments whose maximum lengths during solar disk passage are larger than 70° do not follow the Spörer sunspot law: there is no equatorward and no poleward drift. The latitudinal migration of high‐latitude solar filaments whose maximum lengths during solar disk passage are larger than 70° is more significant than those of all high‐latitude solar filaments and high‐latitude solar filaments whose maximum lengths during solar disk passage are less than or equal to 70°: there is a poleward migration from the latitude of about 50° to 70° and an equatorward migration from the latitude of about 70° to 50° of all high‐latitude solar filaments and high‐latitude solar filaments whose maximum lengths during solar disk passage are less than or equal to 70° and there is a poleward migration from the latitude of about 50° to 80° and an equatorward migration from the latitude of about 80° to 50° of high‐latitude solar filaments whose maximum lengths during solar disk passage are larger than 70°. (2) The statistical characteristics of latitudinal distribution of solar filaments whose maximum lengths during solar disk passage are larger than 70° is different from those of all solar filaments and solar filaments whose maximum lengths during solar disk passage are less than or equal to 70° (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Substorm morphology of > 100 keV protons

The latitudinal morphology of > 100 keV protons at different local times has been studied as a function of substorm activity. A characteristic pattern is found: during quiet-times there is an isotropic zone centred around 67° near midnight, but located on higher latitudes towards dusk and dawn. This zone moves slightly equatorward during the substorm growth phase. During the expansive phase the precipitation spreads poleward apparently to ~ 71° near midnight. The protons are precipitated over a large local time interval on the nightside, but the most intense fluxes are found in the pre-midnight sector. A further poleward expansion, to more than 75° near midnight, seems to take place late in the substorm. Away from midnight, the expansion reaches even higher latitudes. During the recovery phase the intensity of the expanded region decreases gradually; the poleward boundary is almost stationary if the interplanetary magnetic field (IMF) has a northward component and no further substorm activity takes place. Mainly protons with energy below ~ 500 keV are precipitated in the expanded region. On the dayside no increase in the precipitation rates is found during substorm expansion, but late in the substorm an enhanced precipitation is found, covering several degrees in latitude. The low-latitude anisotropic precipitation zone is remarkably stable during substorms. A schematic model is presented and discussed in relation to earlier results.     

Subsurface Meridional Flow from HMI Using the Ring-Diagram Pipeline

We have determined the meridional flows in subsurface layers for 18 Carrington rotations (CR 2097 to 2114) analyzing high-resolution Dopplergrams obtained with the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). We are especially interested in flows at high latitudes up to 75^° in order to address the question whether the meridional flow remains poleward or reverses direction (so-called counter cells). The flows have been determined in depth from near-surface layers to about 16 Mm using the HMI ring-diagram pipeline. The measured meridional flows show systematic effects, such as a variation with the B ₀-angle and a variation with central meridian distance (CMD). These variations have been taken into account to lead to more reliable flow estimates at high latitudes. The corrected average meridional flow is poleward at most depths and latitudes with a maximum amplitude of about $20~\mathrm{m\,s}^{-1}$ near 37.5^° latitude. The flows are more poleward on the equatorward side of the mean latitude of magnetic activity at 22^° and less poleward on the poleward side, which can be interpreted as convergent flows near the mean latitude of activity. The corrected meridional flow is poleward at all depths within ±?67.5^° latitude. The corrected flow is equatorward only at 75^° latitude in the southern hemisphere at depths between about 4 and 8 Mm and at 75^° latitude in the northern hemisphere only when the B ₀ angle is barely large enough to measure flows at this latitude. These counter cells are most likely the remains of an insufficiently corrected B ₀-angle variation and not of solar origin. Flow measurements and B ₀-angle corrections are difficult at the highest latitude because these flows are only determined during limited periods when the B ₀ angle is sufficiently large.     

Observed connections between apparent polar cap features and the instantaneous diffuse auroral oval

Images of the instantaneous nightside auroral distribution reveal that at times the orientation of auroral oval arcs changes to become characteristic of polar cap arcs. These connecting arcs all terminate in the diffuse aurora in the midnight sector, and their separation from the equatorward boundary of the diffuse aurora generally increases away from the midnight termination. The occurrence of these features requires a northward interplanetary magnetic field (positive B_z) as well as low magnetic activity. The existence of connecting arcs and the observation that they are at times the poleward boundary of weak diffuse emission indicate that the poleward boundary of auroral emissions can be significantly modified during non-substorm periods. Such a distortion implies that there can be a modification of the standard convection pattern in the magnetosphere during periods of positive B_z to produce expanded regions of sunward convection in the high latitude ionosphere.     

Response of the polar cap boundary and the current system to changes in IMF observed from the Magsat satellite in the southern hemisphere during summer

The magnetic field vector residuals observed from the Magsat satellite have been used to obtain the dependence of the polar cap boundary and the current system on IMF for quiet and mildly disturbed conditions (K_p ⩽ 3 +). The study has been carried out for the summer months in the Southern Hemisphere. “Shear reversals” (SRs) in vector residuals indicative of the infinite current sheet approximation of the field-aligned currents (FACs) indicate roughly the polar cap boundary or the poleward boundary of the plasma sheet. This is also the poleward edge of the region 1 FACs. The SR is defined to occur at the latitude where the vector goes to minimum and changes direction by approximately 180°.It is found that SRs mainly occur when the interplanetary magnetic field (IMF) has a southward-directed B_z- component and in the latitude range of about 70°–80°. SRs in the dusk sector occur predominantly when the azimuthal component B_y is positive and in the dawn sector when B_y is negative, irrespective of the sign of B_z These results agree with the known merging process of IMF with magnetopause field lines. When SRs occur on both dawn and dusk sectors, the residuals over the entire polar cap are nearly uniform in direction and magnitude, indicating negligible polar currents. Similar behaviour is observed during highly disturbed conditions usually associated with large negative values of B_z.Forty-one Magsat orbits with such SRs are quantitatively modelled for preliminary case studies of the resulting current distribution. It is found that SRs, in the plane perpendicular to the geomagnetic field, for the current vectors and the magnetic vector residuals (perturbations relative to the unperturbed field) occur at almost the same latitudes. The electrojet intensities range from 1.2 × 10⁴ to 6.5 × 10⁵ A (amperes). A preliminary classification of polar cap boundary crossings characterized by vector rotations rather than SRs also shows that they tend to occur mainly for negative B_z.     

The response of the mid-latitude thermospheric wind to magnetic activity

Observations of the thermospheric wind at a mid-latitude station have been made using a Fabry-Perot interferometer to measure the Doppler shift of the nighttime OI emission at 630 nm. The results from 12 summer nights show that the zonal wind has a distinct feature associated with magnetic activity. The zonal wind first reverses and becomes westward. The maximum strength of the westward wind, its duration, and the maximum strength of the subsequent eastward wind all increase with increasing magnetic activity. The meridional wind is less consistent in its behaviour. It is normally equatorward but during magnetic activity it can increase, decrease, or even reverse, although it is consistently equatorward and of increased strength after 02.00 L.T. The initial reversal of the zonal wind is consistent with changes in the wind expected as a result of convective electric fields penetrating to mid-latitudes indicating that these electric fields modify the mid-latitude wind pattern before effects due to auroral heating reach mid-latitudes. The reversal of the zonal wind back to eastward may also be the result of electric field effects. The large variability of the meridional wind, to the extent that it becomes poleward at times, indicates the importance of wind sources equatorward of the observatory.