Google Scholar: https://scholar.google.fr/citations?user=gxcbU_EAAAAJ&hl=en
ORCID iD: https://orcid.org/0000-0003-0447-1531
Web of Science: https://publons.com/researcher/1738676/rajkumar-hajra/
Scopus Author ID: https://www.scopus.com/authid/detail.uri?authorId=23491818900
Ionospheric study: development of ionospheric model for Indian region
My Ph. D. thesis work was on ionospheric topics. One of the goals was the empirical modeling of the (northern) crest of the equatorial ionization anomaly (EIA) over a solar cycle (Hajra et al., SWSC 6, 2016). Plane polarized radio signals transmitted from the ETS-2 geostationary satellite to a University of Calcutta, India ground station was used to determine the total electron content (TEC) of the ionosphere over Calcutta from 1980 to 1990. TEC, defined as the total number of free, thermal electrons present in a column of one meter squared cross section along the path between a radio transmitter and a receiver, is an important ionospheric state parameter. This study was based on “quiet” intervals when geomagnetic Dst index was > -50 nT and the TEC information was used to develop an empirical model of the northern EIA crest. Since the region around the EIA crest is most vulnerable for transionospheric communication and navigation links systems disruptions, this baseline can be useful to assign risks during geomagnetic active intervals. We also performed ionospheric studies for normal electrojet and counter electrojet conditions, and geomagnetic storms (Chakraborty and Hajra, BASI 35, 2007; AnnGeo 26, 2008; AnnGeo 27, 2009; JASTP 72, 2010; Chakraborty et al., IJRSP 37, 2008; Hajra et al., RS 44, 2009). Electrojet is an intense ionospheric current system flowing in the eastward direction over the geomagnetic equator and it controls the EIA variability. It reverses direction during morning and afternoon counter electrojet intervals. We identified and quantified the EIA perturbations during the counter electrojet events. Ionospheric irregularities were studied using the techniques of equatorial spread-F and ionospheric scintillation during geomagnetically quiet and disturbed conditions (Hajra et al., JASTP 72, 2010; JGR 117, 2012; Hajra and Chakraborty, JASTP 73, 2011; Chakraborty et al., RS 47, 2012).
HILDCAAs as a consequence of HSSs
We were the first to study the solar cycle dependence of “high-intensity long-duration continuous AE activity” (HILDCAA) events (Hajra et al., JGR 118, 2013; JASTP 121, 2014; JGR 119, 2014). These magnetospheric/ionospheric events were shown to be primarily (94%) associated with solar wind high-speed (Vsw ~550-800 km s-1) streams (HSSs) emanating from solar coronal holes. These HILDCAA events are the major source of solar wind energy injection into the magnetosphere (Hajra et al., JGR 119, 2014). These intervals of continuous substorms/injection events lasting for days to weeks inject more energy into the magnetosphere than do interplanetary coronal mass ejections (ICMEs) during the maximum phase of the solar cycle.
Magnetospheric relativistic electron acceleration during HILDCAAs
We showed that for an entire solar cycle (SC23) there was a one-to-one correlation between HILDCAAs and the acceleration of relativistic “killer” magnetospheric electrons (these high energy electrons can damage satellite electronics), solving a long-time mystery of why relativistic electron fluxes were highest during the solar cycle declining phase (Hajra et al., GRL 41, 2014; EPS 67, 2015; ApJ 799, 2015). The current scenario among both space plasma theorists and experimentalists is that substorms and convection events within HILDCAAs cause sporadic injection of ~10-100 keV electrons into the nightside sector of the magnetosphere. The anisotropic electrons generate electromagnetic chorus waves, which in turn interact with the upper energy end of the electron spectrum, the ~100 keV particles, to accelerate them to relativistic ~MeV energies.
HILDCAAs as the magnetospheric relativistic electron predictor
It was found that the relativistic electrons are accelerated in steps (Hajra et al., ApJ 799, 2015). First: the lowest energies are accelerated to higher energies and then those to even higher energies. Empirically it was found that after HILDCAA onset, it takes the HILDCAA chorus ~1 day to accelerate E > 0.6 MeV electrons to detectable levels and ~2.5 days for the E > 4.0 MeV electrons to be accelerated. This indicates that the electrons are accelerated in a “boot strap” method. This finding will allow researchers to predict when relativistic electrons will be populating the magnetosphere. Predicting space weather hazards is a major societal importance of understanding space weather science.
How are the relativistic magnetospheric electrons lost? This question was addressed in Hajra & Tsurutani (Elsevier Inc. 2018). Interplanetary heliospheric plasma sheet impacts on the magnetosphere are the cause of the relativistic electron loss mechanism. These are slow solar wind structures. When they impinge upon the magnetosphere, the outer zone magnetospheric electrons “disappear”, typically within an hour. Interplanetary shocks were found to do the same thing, but the compression of shocks is found to be less of a factor than the slow solar wind high densities. Where do these particles disappear to? This is currently under investigation. An old idea is “magnetopause shadowing” where the particles gradient drift out the dayside magnetopause and are lost to the solar wind. However, it has been postulated that if some of the particles precipitate into the subauroral ionosphere, the energy deposition could trigger weather patterns in the Earth’s atmosphere.
Ionospheric response to the HSS was explored in Hajra et al. (GRL 44, 2017). It was reported, for the first time, that the near-equatorial topside ionosphere expanded in altitude and became hotter by the influence of a HSS. Possible role of plasmaspheric wave activity on the ionospheric heating was discussed.
Extremely intense substorms
We made a major finding in studying very intense substorms (supersubstorms) (Hajra et al., JGR 121, 2016). It was shown that these supersubstorms often occur within intense magnetic storms. Thus, it is possible that it is these substorms rather than any particular feature of magnetic storms that are the major danger for power grids. Using the fastest satellite imaging of the auroras during several supersubstorms, we showed that the auroral features changed faster than the ~1 min cadence of the UV images, calling for more capable instrumentation for the future (Hajra & Tsurutani, ApJ 858, 2018). If the aurora truly sweeps long distances in very short time spans, the dB/dt of the currents in the ~100 km altitude ionosphere will indeed be extremely high. This is speculated as a possible cause for geomagnetically induced currents on the ground.
Cometary plasma and solar wind interaction study by ESA/Rosetta
We are working on the cometary plasma characteristics of the comet 67P/Churyumov-Gerasimenko using the data collected by the Rosetta orbiter of the European Space Agency (ESA). Rosetta was an unprecedented exploratory mission that monitored in-situ a comet for more than 2 years. The aim of our present work is to understand and characterize the cometary plasma interaction with the solar wind and to compare that with the solar wind interaction with the Earth. While the Earth has an intrinsic magnetic field, non-magnetized objects like the comets possess an ionosphere (a partially ionized atmosphere) that interacts with the solar wind through the formation of an induced magnetosphere. The induced magnetosphere is characterized by the draping of interplanetary magnetic field lines around the conducting object. In Hajra et al. (MNRAS 480, 2018) the interplanetary characteristics and impacts of CIRs on the near cometary plasma are explored in detail. In Hajra et al. (A&A 607, 2017), we reported, for the first-time, in-situ observations of the plasma responses to a cometary brightness outburst while the previous cometary outburst studies are based on remote and/or fly-by observations and modeling. The dynamics of the unmagnetized cometary plasma and associated wave activities are studied for the first time in Henri et al. (MNRAS 469, 2017) and Hajra et al. (MNRAS, 475, 2018).