OVRO-LWA Solar Data Products

Introduction

OVRO-LWA is an all-sky radio imager by design and hence, in principle, can observe the Sun as long as it is above the horizon. However, the array is located in a valley surrounded by mountains, and hence the Sun is not visible from the array when its line-of-sight from the array passes through the mountains. Additionally, when the Sun is at very low elevations, the performance of the array degrades, and the data gets increasingly affected by terrestrial radio emission. Keeping this in mind, the Sun is observed with the OVRO-LWA when the solar elevation is greater than approximately 15 degrees. This translates to about 7-14 hours of observations depending on the season.

The highest time-frequency resolution at which data can be obtained with the OVRO-LWA in a regular manner is 1 ms and 24 kHz, respectively. However, depending on the observation mode as well as due to data rate limitations, the actual available time-frequency resolution can vary. Figure 1 summarizes the different data products we produce. The later sections will give a more detailed description and usage examples.

Level 0 - Raw data from the instrument

OVRO-LWA, in general, operates multiple observing modes simultaneously. This is achieved by passing the raw data stream from the 352 antennas through multiple data handling processes, with each process handling an observation mode. The figure above summarizes the three data streams relevant to the solar data. These data streams are:

• Beamforming Dynamic Spectroscopy Data: The OVRO-LWA beamformer uses the 256 antennas in the core region to form a synthesized beam of more than 1 degree in size that tracks the Sun from sunrise to sunset. This permits a continuous record of the full-Stokes total flux (without spatial resolution) of the Sun (a dynamic spectrum) with 24 kHz frequency resolution (3072 frequency channels from 13.4-86.9 MHz) and as low as 1 ms time resolution. For regular solar observations, we write data out with full spectral resolution and 64 ms time resolution. These raw data are permanently stored in the Hierarchical Data Format (HDF) on our data server. The level-0 spectrograms are generated in true real-time (with <1-s of latency). Quicklook spectrograms, in PNG format, are displayed on our observing status page. Note that the level-0 spectrograms are already partially calibrated. They have close-to-true flux scale, but they contain the non-solar background and have not been corrected for the primary beam response.
• Standard Interferometric Imaging (also known as "slow visibilities"): In this mode, the entire 352-element array is interferometrically correlated to provide visibilities for imaging at all 3072 frequencies at 10-s time resolution. This mode is ideal for high-dynamic-range, high-fidelity imaging of relatively slowly varying emissions, such as active regions, coronal holes, and incoherent emission from coronal mass ejections. The data is stored in CASA's measurement set format (see this link for more details). With a daily data rate of 17 TB, we currently do not have the capacity to store all the raw data. Hence, they are stored in a 7-day rolling "buffer." Extremely interesting/important events will be copied and saved on a case-by-case basis. Our automatic pipeline processes the data into spectral images at a reduced cadence and spectral resolution (see next section).
• Bursty Interferometric Imaging (also known as "fast visibilities"): In this mode, a subset of 48 antennas (chosen to include mainly outer antennas to maintain good spatial resolution) is interferometrically correlated to provide visibilities for imaging at 768 frequencies (96 kHz frequency resolution) at 0.1-s time resolution. This mode is ideal for imaging rapidly varying emission with fine spectro-temporal structures, such as type II and type III bursts with spectral fine structures. The trade-off is the imaging dynamic range and fidelity. The data is also stored in CASA's measurement set format. With a daily data rate of 9 TB, we currently do not have the capacity to store all the raw data. Hence, they are stored in a 3-day rolling "buffer." Extremely interesting/important events will be copied and saved on a case-by-case basis. A pipeline for processing the data into spectral images in a "triggered" mode is currently under construction.

Level 1.0 - Calibrated Spectrogram and Spectral Imaging Data

The resulting dynamic spectrum data is provided in standard FITS format.

Our automated data pipeline processes the level 0 raw data by applying appropriate calibrations (complex gain, bandpass, and absolute flux scale), performing self-calibration, and finally, synthesis imaging to convert them into spectral images. Although OVRO-LWA obtains fully polarized data, for the current data release (v1.0), we provide Stokes I data products only. A pipeline for processing polarized data products is being constructed and tested. We are preparing a paper on the pipeline, which will provide more details of the data processing steps.

The images are generated in near real-time (with several minutes of latency). Quicklook images and movies at selected frequency bands, in PNG format, are displayed on our observing status page. The spectral image data themselves are stored and provided in HDF format. We also provide software to convert the HDF5 files to FITS file format. The produced FITS files, apart from the multi-frequency data, also contain a table containing the frequencies corresponding to the multi-frequency images and the instrumental resolution at each frequency. It also contains other parameters necessary to convert the images to level 1.5, which might be useful for some scientific purposes. These images are in heliocentric coordinates and typically have a dynamic range of >300 (the dynamic range of an image refers to the ratio of the image maximum and the image "noise," usually represented by rms of a source-free region). The FITS images can be directly loaded into Python or SSWIDL using standard techniques.

Below we describe each of these level 1 data products.

• Imaging data products:
  • Frequency integrated images: 12 images, with center frequencies of 34, 39, 43, 48, 52, 57, 62, 66, 71, 75, 80 and 84 MHz, are provided. The images are produced by integrating 10s and approximately 5 MHz of data. The cadence of the images vary from 10s to sometimes a minute.

• • High frequency resolution images: 144 images, with frequencies ranging from 32 - 88 MHz, with frequency integration of 416 kHz. The imaging cadence and time integration is the same as that of the frequency integrated images.

• Dynamic spectrum data product: The dynamic spectrum data are provided as standard FITS tables containing the frequency list, list of times, and the Stokes I flux density in SFU.

Level 1.5 - Images

Low radio frequency waves, during propagation, get significantly affected by ionospheric refraction. Ionospheric refraction often results in source shifts only, without any change of the source morphology and flux density. In this case, it can be shown that the source shift is inversely proportional to the square of the observation frequency. Here this dependence is used to try to correct for refraction. The proportionality constant is directly dependent on the gradient of the ionospheric total electron count along the line-of-sight towards the source. When the sun is quiet, the imaging dynamic range is sufficiently high to easily see the quiet sun disc. Hence the ionospheric source shift can be determined by comparing the observed center of the solar disc with the optical location of the Sun. The source shifts at multiple frequencies can be fitted to obtain the ionospheric parameters, and then can be applied to the images at other frequencies, where the solar disc is not well observed due to dynamic range limitations.