Probing the Universe with Space Based Low-Frequency Radio Measurements

Abstract

Due to Earth’s ionosphere, it is not possible to image the sky below 10 MHz. Any waves below this cutoff frequency are absorbed by the plasma in Earth’s ionosphere, whose free electron density determines the cutoff. A constellation of small spacecraft above the ionosphere could enable radio imaging from space at frequencies below this cutoff, but the logistics and costs of doing this imaging using multiple satellites that are kilometers apart in a precise enough manner to form a radio array has until recently been unfeasible. With the lowering costs and increasing reliability of smallsats, the use of radio arrays in space is finally set to open up this new window through which we may observe the universe in a new light. For complex sources in the sky, analytical formulas are not enough to predict array performance; full simulations must be done to evaluate potential array configurations. Simulated outputs must be compared to a realistic input model to make sure that a given array configuration can meet its defined scientific requirements. Space-based arrays also introduce additional challenges in understanding novel data processing and errors from location retrieval of the receivers and budgeting for data transmission. In this thesis I demonstrate the feasibility for different space-based radio arrays by simulating their performance under realistic conditions. I outline the science goals involving radio imaging below 10 MHz for a range of solar, astrophysical, and magnetospheric targets. I then outline different strategies for creating synthetic apertures in space that are well suited for each of these targets. I describe the calculations needed for each style of correlation and create a data processing and science analysis pipeline for showcasing the imaging performance of each simulated array. I show that the SunRISE and RELIC array concepts are both able to meet their main scientific goals of localizing solar radio bursts and mapping radio galaxies respectively. I describe a novel way in which I use magnetohydrodynamic simulations of a solar eruption alongside real radio data to predict the sky brightness patterns of the radio bursts for input to the SunRISE pipeline across different theories of particle acceleration. This technique provides initial predictions of the location of solar type II burst generation in a coronal mass ejection that SunRISE can potentially confirm. I also demonstrate the feasibility of a lunar near side array powerful enough to image the Earth’s synchrotron emission, along with a zoo of brighter auroral emissions. Synchrotron measurements would provide a unique proxy measurement of the global energetic electron distribution in the Earth’s radiation belts. Such an array could also pinpoint the location of brighter transient events such as Auroral Kilometric Radiation with high precision, providing local, small scale electron data in addition to global data. The time finally seems ripe for low frequency radio astronomy to make its move to outer space. Increased feasibility of small satellites is a huge game changer for the entire space industry, incentivizing mission designs that can take advantage of the distributive nature of multiple small inexpensive spacecraft to do the jobs traditionally done, or unable to be done, by larger, more costly single spacecraft. In that same spirit, this work acts as a helpful starting point for the general mission design, data processing, and science analysis required for distributed space-based radio arrays.