Separating Nightside Interplanetary and Ionospheric Scintillation with LOFAR
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Separating Nightside Interplanetary and Ionospheric Scintillation with LOFAR R.A. Fallows ASTRON - the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, the Netherlands [email protected] M.M. Bisi RAL Space, Science & Technology Facilities Council - Rutherford Appleton Laboratory, Harwell, Oxford, Oxfordshire, OX11 0QX, United Kingdom [email protected] B. Forte Dept of Electronic and Electrical Engineering, University of Bath, Bath, BA2 7AY, United Kingdom [email protected] Th. Ulich Sodankyl¨aGeophysical Observatory, T¨ahtel¨antie62, FIN-99600 Sodankyl¨a,Finland A.A. Konovalenko Institute of Radio Astronomy, 4 Chervonopraporna str., 61002 Kharkov, Ukraine G. Mann Leibniz-Institut fr Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany and C. Vocks Leibniz-Institut fr Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany arXiv:1608.04504v1 [astro-ph.IM] 16 Aug 2016 ABSTRACT Observation of interplanetary scintillation (IPS) beyond Earth-orbit can be challenging due to the necessity to use low radio frequencies at which scintillation due to the ionosphere could confuse the interplanetary contribution. A recent paper by Kaplan et al (2015) presenting observations using the Murchison Widefield Array (MWA) reports evidence of night-side IPS on two radio sources within their field of view. However, the low time cadence of 2 s used might be expected to average out the IPS signal, resulting in the reasonable assumption that the scintillation is more likely to be ionospheric in origin. To verify or otherwise this assumption, this letter uses observations of IPS taken at a high time cadence using the Low Frequency Array (LOFAR). Averaging these to the same as the MWA observations, we demonstrate that the MWA result is consistent with IPS, although some contribution from the ionosphere cannot be ruled out. These LOFAR observations represent the first of night-side IPS using LOFAR, with solar wind speeds consistent with a slow solar wind stream in one observation1 and a CME expecting to be observed in another. Subject headings: scattering | Sun: coronal mass ejections (CMEs) | Sun: solar wind | Sun: solarter- restrial relations 1. Introduction in the Netherlands but with a number of stations across Europe is capable of observing frequencies The use of interplanetary scintillation (IPS - in the range 10{250 MHz, including full coverage Clarke (1964), published by Hewish et al. (1964)) of those used by K2015. It has on-line beam- to observe the solar wind beyond Earth-orbit can forming capabilities and the ability to record data be a challenging proposition with few papers ded- per station, enabling it to be used as a large collec- icated to the subject. Early papers described ob- tion of individual telescopes, with baselines rang- servations of the level of scintillation of B0531+21 ing from ∼50 metres to ∼1,500 kilometres (as of ◦ out to 180 from the Sun (e.g. Armstrong & Coles early 2016), in similar fashion to more-traditional 1978). More recently, the Ukrainian URAN and systems. Several observations of IPS have been UTR-2 telescopes have been used to estimate so- carried out using LOFAR since full operations lar wind speeds beyond Earth orbit from obser- commenced in 2012 (initial observations are pre- vations of IPS (e.g. Fal'Kovich et al. 2010; Olyak sented in Fallows et al. (2013); Bisi et al. (2016)) 2013). One of the challenges is the necessity to use and irregular monitoring of ionospheric scintilla- low radio frequencies where the ionosphere could tion has been performed since 2014 (Fallows et al. be the dominant source of any scintillation seen. in prep.). Regular observations of IPS inside of Earth-orbit, The Kilpisj¨arviAtmospheric Imaging Receiver by contrast, are usually taken during local day- Array (KAIRA; McKay-Bukowski et al. (2014)), time hours and observatories such as the Institute a station built using LOFAR hardware in arctic for Space-Earth Environmental Research (ISEE), Finland, has been routinely monitoring the iono- Japan, (e.g. Kojima & Kakinuma 1987), and Ooty, sphere, including ionospheric scintillation, since India (e.g. Manoharan & Ananthakrishnan 1990), 2012 (e.g. Fallows et al. 2014): The ionospheric IPS arrays use a higher observing frequency. scintillation conditions above KAIRA are natu- In a recent letter, Kaplan et al. (2015), here- rally more severe than above LOFAR: At auroral inafter referred to as K2015, presented wide- latitudes, refractive index gradients due to field- field \snapshot" imaging observations using the line elongated ionisation structures are stronger Murchison Widefield Array (MWA; Lonsdale et al. than in the case of middle latitudes structures. (2009) and Tingay et al. (2013)) in which they These observations can, therefore, be used to ver- claimed to see, from successive images, IPS on ify the effects of periods of strong ionospheric scin- flux measurements of two sources within the field tillation. of view, despite a 2 s time cadence between im- In this letter, we use observations of interplan- ages which might be expected to average out the etary and ionospheric scintillation from both of IPS signal. The observations were also taken at these arrays to provide a comparison with the night, with the scintillating sources at solar elon- K2015 result. gations of ∼110-115◦, potentially indicating that the scintillation seen could be ionospheric in ori- 2. Observations and Results gin. Hence, the question arises whether or not the interplanetary medium is the dominant source The observations presented here are analysed of the stochastic variations seen in the received with the aim of answering three specific questions:- signal. K2015 goes into significant detail to allay concerns, but the current lack of a high time- • Is IPS averaged out with an integration time cadence capability (although post-processing of of 2 s? voltage-capture data is now underway) does not allow for a proper evaluation of the scintillation • Is IPS observed beyond Earth-orbit, and seen. The use of high time-cadence observations could it be confused with ionospheric scin- can help to ascertain the combination of IPS and tillation? ionospheric scintillation contributions to the ob- • Which power spectra, those from IPS or served signal intensities. those from ionospheric scintillation, are The Low Frequency Array (LOFAR; van Haar- more consistent with the K2015 result? lem et al. (2013)), a modern radio telescope based 2 In November 2015 a series of observations were taken under an ionospheric scintillation monitor- CS401-CS011: 00:38 to 01:48UT ing project, LC5 001, to observe both 3C48, a very 1.0 Baseline: 1080m compact source known as one of the strongest scin- Azimuth: -92 degrees tillators from plasma structures in the interplane- 0.8 Velocity: 51m/s tary medium, and Cassiopeia A, a relatively broad 0.6 source known to scintillate at low radio frequen- cies from plasma structures in the ionosphere, but 0.4 too broad to scintillate from plasma structures in Correlation the interplanetary medium. LOFAR was set up to 0.2 record beam-formed data from each station indi- vidually (\Fly's Eye" mode - see Stappers et al. 0.0 (2011)) over the frequency range 110{178 MHz, 0.2 with a frequency resolution of 12 kHz and a time 40 30 20 10 0 10 20 30 40 Time lag, s cadence of approximately 0.01 s. The data were RS409-RS210: 00:38 to 01:48UT averaged in post-processing to a final frequency 1.0 Radial baseline: 80km resolution of 195 kHz and time resolution of ap- Tangential baseline: -5km proximately 0.1 s. The stations of the LOFAR 0.8 Velocity: 212km/s \core", a dense group of stations covering an area with a diameter of approximately 3 km, were used 0.6 to observe Cassiopeia A; remaining stations across 0.4 the Netherlands and internationally were used to Correlation observe 3C48. 0.2 At this time, 3C48 was at a solar elongation of approximately 157◦ and scintillation was evi- 0.0 dent upon inspecting the data. This is a greater 0.2 10 5 0 5 10 elongation than the K2015 observations and any Time lag, s IPS is expected to be weaker as a consequence. RS306-RS205: 00:38 to 01:48UT The origin of the 3C48 scintillation is confirmed 1.0 Baseline: 11092m using a cross-correlation analysis: In the case of Azimuth: -70 degrees the ionosphere, bulk flows of 10s to 100s of me- 0.8 Velocity: 209m/s tres per second lead to a time delay of several, 0.6 and possibly 10s, of seconds over the short base- lines between stations within the LOFAR core (for 0.4 baselines with a component aligned with the iono- Correlation spheric bulk flow). The solar wind flows much 0.2 faster and even a slow solar wind stream of ap- proximately 350 km s−1 leads to time delays of less 0.0 than a second between any pair of LOFAR remote 0.2 stations, with baseline lengths of tens of kilome- 80 60 40 20 0 20 40 60 80 Time lag, s ters. Correlation of IPS is also expected over in- ternational station baselines of hundreds of kilo- Fig. 1.| Plots of auto- (dashed and dotted meters. lines) and cross-correlation (solid line) functions of In order to calculate power spectra and corre- time series' calculated from the observations of 8 lation functions, time series' were first obtained November 2015, over the entire duration of the ob- by taking the median over the pass-band of inter- servations. Top: Cassiopeia A data from core sta- est from the data received by each station. To tions CS401 and CS011; middle: 3C48 data from match the data presented in K2015, only 32 MHz remote stations RS409 and RS210; bottom: 3C48 of the recorded bandwidth was used, centred on data from remote stations RS306 and RS205.