Nurul Shazana Abdul Hamid

Relationship between the equatorial electrojet and global Sq currents at the dip equator region

The equatorial electrojet (EEJ) is a strong eastward ionospheric current flowing in a narrow band along the dip equator. In this study, we examined the EEJ-Sq relationship by using observations at six stations in the South American, Indian, and Southeast Asian sectors. The analysis was carried out with data on geomagnetically quiet days with Kp ≤3 from 2005 to 2011. A normalization approach was used because it yields more accurate results by overcoming the uncertainties due to latitudinal variation of the EEJ and Sq. A weak positive correlation between the EEJ and Sq was obtained in the Southeast Asian sector, while weak negative correlations were obtained in the South American and Indian sectors. EEJ-Sq relationship is found to be independent of the hemispheric configuration of stations used to calculate their magnetic perturbations, and it also only changed slightly during low and moderate solar activity levels. These results demonstrate that the Southeast Asian sector is indeed different from the Indian and South American sectors, which is indicative of unique physical processes particularly related to the electro-dynamo. Furthermore, we also demonstrate that the definition of the EEJ, that is, the total current or enhanced current, can significantly affect the conclusions drawn from EEJ-Sq correlations.

The Hurst exponents of the geomagnetic horizontal component during quiet and active periods

The focus in this paper is mainly to characterize the fractal properties denoted by the Hurst exponent of geomagnetic field during quiet and active periods of geomagnetism activity. Analysis is made on the time series of the geomagnetic horizontal component data, H, which were acquired by the Magnetic Data Acquisition System (MAGDAS) developed by the Space Environment Research Center (SERC) of Kyushu University in Japan. The data set covers the quiet and active periods in August 2005 at the equatorial stations of Cebu and Davao in the Philippines. This month had very intense geomagnetic storm with the Dstmin, Kp and Ap indices for the quietest and active days of the month are −20 nT, 2, 6 and −216 nT, 9, 110 respectively. For both stations, the data sampling interval is 1 second for each time periods or one day giving a sample size of 86,400. Using the fast Fourier transform, the power law of the data time series was obtained in the shorter time periods of 10 minutes to 6 hours. This observed power spectrum law exhibit scaling with corresponding Hurst exponents of 0.3–0.5 for quiet periods and 0.5–0.7 for active periods for both stations. For comparison, other fractal techniques were performed which are rescaled range analysis (RS) and detrended fluctuation analysis (DFA) on the data set, and the same ranges of the Hurst exponents were obtained. The work also includes simulation of the similar sized data sets by generating the fractional Brownian motions (FBM) with the Hurst exponents of 0.3–0.7. Our results show that the H component at these equatorial stations is a fractal in nature and become more persistent during active periods of geomagnetism activity. Thus, the Hurst exponent could be used to characterize the geomagnetic time series during quiet and active periods.

Empirical model of equatorial electrojet based on ground-based magnetometer data during solar minimum in fall

In this study, we constructed an empirical model of the equatorial electrojet (EEJ), including local time and longitudinal dependence, based on simultaneous data from 12 magnetometer stations located in six longitude sectors. An analysis was carried out using the equatorial electrojet index, EUEL, calculated from the geomagnetic northward H component. The magnetic EEJ strength is calculated as the difference between the normalized EUEL index of the magnetic dip equator station and the normalized EUEL index of the off-dip equator station located beyond the EEJ band. Analysis showed that this current is always strongest in the South American sector, regardless of local time (LT), and weakest in the Indian sector during 0900 and 1000 LT, but shifted to the African sector during 1100 to 1400 LT. These longitude variations of EEJ roughly follow variations of the inversed main field strength along the dip equator, except for the Indian and Southeast Asian sectors. The result showed that the EEJ component derived from the model exhibits a similar pattern with measured EEJ from ground data during noontime, mainly before 1300 LT.

Scaling and Fractal Properties of the Horizontal Geomagnetic Field at the Tropical Stations of Langkawi and Davao in February 2007

We investigate the scaling and fractal properties of the horizontal component of the geomagnetic field time series acquired by the Magnetic Data Acquisition System (MAGDAS) of the Space Environment Research Center (SERC) of Kyushu University in Japan. The data set covers the quiet period of geomagnetic activity in February 2007 at the near equatorial stations of Langkawi in Malaysia and Davao in the Philippines. These data were sampled every minute for 28 days giving a sample size of 40,320. The power spectra of the time series are obtained, showing dominant periodicities at 24, 12, 8 and 6 hours due to the effect of the sun on the geomagnetic field. The power spectra also indicate scaling with Hurst exponents of 0.50–0.77 in the period of 10 minutes to 6 hours. Then, rescaled range analysis and detrended fluctuation analysis are performed, producing similar ranges of Hurst exponents. Finally, these fractal methods are used to determine the Hurst exponents of similar sized data sets artificially generated...

Comparison of ionospheric profile parameters with IRI-2012 model over Jicamarca

We used the hourly ionogram data obtained from Jicamarca station (12° S, 76.9° W, dip latitude: 1.0° N) an equatorial region to study the variation of the electron density profile parameters: maximum height of F2-layer (hmF2), bottomside thickness (B0) and shape (B1) parameter of F-layer. The period of study is for the year 2010 (solar minimum period).The diurnal monthly averages of these parameters are compared with the updated IRI-2012 model. The results show that hmF2 is highest during the daytime than nighttime. The variation in hmF2 was observed to modulate the thickness of the bottomside F2-layer. The observed hmF2 and B0 post-sunset peak is as result of the upward drift velocity of ionospheric plasma. We found a close agreement between IRI-CCIR hmF2 model and observed hmF2 during 0000-0700 LT while outside this period the model predictions deviate significantly with the observational values. Significant discrepancies are observed between the IRI model options for B0 and the observed B0 values. Specifically, the modeled values do not show B0 post-sunset peak. A fairly good agreement was observed between the observed B1 and IRI model options (ABT-2009 and Bill 2000) for B1.