Archana Bhattacharyya

Dynamics of equatorial F region irregularities from spaced receiver scintillation observations

Spaced receiver observations of amplitude scintillations on a 244 MHz signal, at an equatorial station, have been used to study random temporal changes associated with the scintillation-producing irregularities and the variability of their motion. The computed drift of the scintillation pattern shows the presence of velocity structures associated with equatorial bubbles in the early phase of their development. On magnetically quiet days, after 22:00 LT, the estimated drifts fall into a pattern which is close to that of the ambient plasma drift. There is considerable decorrelation between the two signals until 22:00 LT. The power spectra of the most highly correlated scintillations recorded by spaced receivers indicate that the associated irregularities are confined to a thin layer on the bottomside of the equatorial F region. This suggests that the convection pattern associated with bottomside irregularities is stable due to the dominance of ion-neutral collisions over ion inertia.

Nighttime equatorial ionosphere: GPS scintillations and differential carrier phase fluctuations

The presence of scintillation-producing irregularities in the nighttime equatorial ionosphere, in the path of Global Positioning System (GPS) signals received at an equatorial station, causes dual-frequency measurements of the differential carrier phase of GPS L1 and L2 signals to have a contribution from phase scintillations on the two signals. Dual-frequency data for fluctuations in the total electron content (TEC) along the path of GPS signals to the equatorial station Ancon (1.5° dip), sampled at a rate of 1 Hz, are used to separate this contribution from the slower TEC variations. Rapid fluctuations in the differential carrier phase, usually on timescales < 100 s, which result from diffraction, are seen to follow the pattern of intensity scintillations on the L1 signal. Intensity scintillations are also related to the variations in TEC which arise from density fluctuations associated with ionospheric irregularities. An approximate version of the transport-of-intensity equation, based on a phase screen description of the irregularities, suggests that a quantitative measure of intensity scintillations may be provided by the derivative of rate of change of TEC index (DROTI), obtained from the second derivative of TEC. This equation also yields the dependence of the scaling factor between DROTI and S4 on the Fresnel frequency. Comparison of DROTI computed from relative TEC data to corresponding S4 indices indicates that there may be lesser uncertainity in a quantitative relation between the two than between the index ROTI, introduced in recent years, and S4. Power spectral analysis of TEC fluctuations and simultaneous intensity scintillations on L1 signal, recorded at Ancon, does not indicate any simple dependence of the scaling factor between DROTI and S4 on the spectral characteristics.

DEDUCING TURBULENCE PARAMETERS FROM TRANSIONOSPHERIC SCINTILLATION MEASUREMENTS

The theoretical framework and experimental methodology used to interpret observations of ionospheric scintillations in terms of geophysical processes are reviewed and recent experimental observations of ionospheric scintillations are discussed in this paper. During the past 15 years significant progress has been made in several areas. In particular, significant advances have been made in theoretical understanding of the strong scintillation regime and the effects of short-term temporal variations of the scintillation producing irregularities on observations made with spaced-receiver geometries in both weak and strong scintillations. This improved understanding of the scintillation process has significantly increased the utility of the technique particularly in the equatorial latitudes where geometrical effects are least important.

Effect of magnetic activity on the dynamics of equatorial F region irregularities

[1]xa0Two different aspects of the effect of magnetic activity on the dynamics of equatorial spread F (ESF) irregularities are studied here using spaced receiver scintillation observations. The first one deals with the question of how magnetic activity affects the generation of ESF irregularities. For this, a parameter designated the “random velocity,” which is a measure of random changes in the irregularity drift velocity, is evaluated from the data. In past studies, this parameter has been found to have large values in the early phase of evolution of ESF irregularities during the postsunset period, with a steep decline to a low value by 22 LT. This behavior is attributed to the decline in the height of the F region. Therefore, a sudden increase in the “random velocity” in the postmidnight period is attributed to an increase in the height of the F region due to the ionospheric zonal electric field turning from westward to eastward due to the effect of magnetic activity, which may also generate fresh irregularities that produce the observed scintillations. This idea has been used to suggest that for two of the magnetically active days considered in the present study the irregularities may be freshly generated in the postmidnight period. The second aspect is the identification of geomagnetically disturbed plasma drifts, which is generally possible only after 22 LT, when the estimated irregularity drift velocities are close to that of the background plasma. The pattern of the estimated drift after 22 LT (3 UT) is found to be well defined for magnetically quiet days with scintillations during a period of a month. This allows the identification of a superimposed westward perturbation in the drift, produced by a disturbance dynamo due to magnetic activity, for all the three events studied here. On 19 February and 1 March 1999, the eastward drift velocities show an identical decrease of about 50 m/s from the undisturbed drift at 0440 UT. On 1 March, the decay phase of the storm sets in later, and the eastward velocity continues to decrease until 0530 UT, turning westward with a maximum decrease of about 80 m/s from the undisturbed drift. On 22 October 1999, which was more disturbed than these two days, the westward perturbation was larger, causing the drift velocity to turn westward around 5 UT and a decrease of nearly 150 m/s from the quiet time drift at 8 UT. The results are in broad agreement with some of the recent empirical models of the evolution, with storm time, of equatorial disturbance dynamo electric fields.

L-band scintillation activity and space-time structure of low-latitude UHF scintillations

[1]xa0Spatial correlation function of intensity scintillation patterns produced by the propagation of a UHF signal through irregularities in the nighttime low-latitude ionosphere is deduced from an analysis of spaced receiver records of such scintillations. This analysis requires that random temporal variations of the irregularity drift speed be taken into account. It is seen from the results that the occurrence of strong scintillations on an L-band signal requires the presence of short (∼20 m) coherence scale lengths in the UHF scintillation pattern obtained in the plane of the receiver. This condition is satisfied near the crest of the equatorial ionization anomaly (EIA) region, but not near the dip equator. In the decay phase of L-band scintillations recorded near the crest of the EIA region, the maximum strength of these scintillations at any point in time is found to be correlated with the magnetic eastward drift speed of the pattern of intensity scintillations on an UHF signal recorded in this region, which is determined mainly by the magnetic eastward drift velocity of the ionospheric irregularites. Dependence of the corresponding strength of UHF scintillations on the drift speed indicates that toward the end of the decay phase of L-band scintillations, the irregularity power spectrum steepens, and the large scale irregularities that remain can cause the UHF signal to be focused in the plane of the receiver, yielding UHF S4-indices greater than one, while focusing of the UHF signal is less evident at earlier times when there is focusing of the L-band signal.

A transmission line analogy for the development of equatorial ionospheric bubbles

The Pedersen conductivity of the conjugate E regions couples to the equatorial F region through geomagnetic field lines and plays an important role in the development of equatorial spread F bubbles. Earlier work has suggested that the coupling between the E and F regions is effected through field-aligned currents (FACs). However, these currents have not yet been explicitly introduced into theoretical models. This paper considers oppositely propagating Alfven waves which are launched by equatorial F region perturbations as carriers of FACs and transverse polarization currents. A transmission line analogy is drawn, with the E region loads at the two ends and the generator in the equatorial F region. The currents which flow through the E regions depend on the plasma density of the propagation medium in which the transmission line is immersed. We conclude that whereas small angles between the solar terminator and the magnetic meridian favor the growth of equatorial bubbles, an increase in the plasma density of the propagation medium and a higher altitude of the equatorial F layer allow greater relaxation of the restriction imposed by the E region conductivities on the growth of equatorial bubbles.

Role of E region conductivity in the development of equatorial ionospheric plasma bubbles

[1]xa0A transmission line analogy, which has been used earlier in a linear theory for the development of equatorial plasma bubbles, is extended to study the effect of E region conductivity on non-linear evolution of the plasma bubbles. For this, a set of mode coupling equations are used to describe non-linear development of equatorial plasma bubbles in the presence of field-aligned currents which couple the equatorial F region with conjugate E regions. For a three mode system, these non-linear equations yield a condition for unstable fixed states. This condition shows that E region resistivity together with F region polariz-ability introduces another time scale in the non-linear evolution of equatorial bubbles, and this is the time scale for discharging the bubbles.

Principal components of quiet time temporal variability of equatorial and low-latitude geomagnetic fields

Diurnal variations of the horizontal component of the geomagnetic field ΔH on International Quiet days of 1999–2012, measured hourly at two stations in the same longitude zone in the Northern hemisphere, near and away from the dip equator, have been subjected to principal component analysis. This technique is also applied to the difference ΔHEEJ of ΔH at these two stations, which is attributed to the equatorial electrojet (EEJ). The first three principal components: PC1, PC2, and PC3, account for 91 - 96% of the variances in the data. Maximum contribution to the quiet day variations in ΔH around its peak in the morning hours at both the stations, and in the EEJ, comes from the day-to day variation of the amplitude of PC1. Patterns of day-to-day variations of PC1 amplitudes for the equatorial station and the EEJ are essentially semiannual modulated by solar EUV flux, superimposed on a longer time scale solar EUV flux-dependent trend. Contributions from PC2 and to a lesser extent from PC3 are seen to be responsible for the absence of semi-annual variations in ΔH in the afternoon hours at the equatorial station. Distribution of amplitudes of PC2 and PC3 for ΔHEEJ for weak electrojet days, shows seasonal features in accordance with greater occurrence of afternoon (morning) counter-electrojet during June (December) solstice. During the extended solar minimum, PC3 amplitudes for ΔH at the equatorial station and for the EEJ display annual variation. Possible sources for these seasonal features in the variations of equatorial ΔH are discussed.

Deterministic retrieval of ionospheric phase screen from amplitude scintillations

In the phase screen theory of ionospheric scintillations the ionospheric irregularities are considered as pure phase objects, and the diffraction pattern produced on the ground, by radio waves propagating through this phase screen, is obtained from conventional Fresnel diffraction theory. In the present paper the inverse problem of retrieving phase variations in the screen from the diffraction pattern on the ground is treated by using the transport-of-intensity equation (TIE), which is derived from the Fresnel diffraction theory result. An approximate version of the TIE is solved to obtain the phase variation in the screen from the intensity distribution on the ground. It is seen that when the phase variations involve spatial scale lengths ∼400 m, phase fluctuations of ∼5 rad can be recovered from intensity variations on the ground, for a 300-MHz signal. For larger spatial scale lengths and higher frequencies this method can be used to retrieve larger phase variations in the screen.

Role of IMF By in the prompt electric field disturbances over equatorial ionosphere during a space weather event

On 7 January 2005 (A p =40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However, the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during 14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) B z condition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during 15:00–15:15 UT, despite IMF B z remaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF B y component under southward IMF B z condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high-latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF B y are fundamentally important to understand the prompt penetration effects over low latitudes.