Following up on our first attempt to visualize the variation in hiss occurrence rate as a function of IMF Clock Angle (variation in IMF By and IMF Bz), I plotted a second set of plots to better see certain features of hiss dependence on IMF. Below is a list of changes that I've made in the data analysis technique and dependencies that I've highlighted in each plot:
Zero Level for IMF By and Bz:
1. I've adjusted the 'zero level' for both By and Bz to ensure large (as much as possible) number of hiss samples going into the plotting of the radial spectrograms.
-> Since By and Bz are hardly ever exactly 0, so I had to set a bound around zero, s.t. if the absolute value of By/Bz for a hiss interval is below this bound, then term it as a "hiss for 0 (very small) By.
-> 'Zero level' for each of By and Bz was set, in most cases, to the half of the mean value of By and Bz respectively over the entire period of hiss data acquisition. This value was empirically chosen to ensure large number of hiss samples at each clock angle.
Averaging IMF By / Bz over the Past Half Hour from the Time of Hiss Occurrence:
2. I've plotted two plots for each category described below, one where I've used IMF By / Bz values representing a hiss event as the exact IMF By / Bz values found at the Time of Hiss Occurrence, and the other plot, where I've used the mean value of IMF By / Bz for past half hour from the time of hiss occurrence to represent the value of IMF By / Bz for a particular hiss event.
Total Hiss Occurrence Rate Plotted at the Center:
3. Total Hiss Occurrence Rate or Total Cumulative Hiss Spectrogram have been plotted at the center of the 3x3 matrix IMF Clock angle plots for comparison purposes.
Variation in By only, Variation in Bz only, and Variation in both By and Bz:
4. I have modified IMF clock angle radial spectrogram plots to depict the 'Variation in Hiss Occurrence Rate as a function of IMF By only, as a function of IMF Bz only, and finally Variation in Hiss Occurrence rate and Cumulative Spectrogram as a function of Both By and Bz.
Other Changes:
5. Colorbar has been kept uniform for all plots and radial frequency dependence has been corrected to show log. dependence.
Plots:
1. Dependence of Hiss Occurrence Rate on IMF By
Above plot makes prominent the two key observations that we made from the more comprehensive IMF Clock Angle plot which included the dependence on IMF Bz as well:
As By grows positive,
a. we see a gap forming in hiss occurrence rate at dusk
b. increase in the hiss occurrence rate is seen in late afternoon/ pre-dusk sector
c. hiss occurrence increases in near and post midnight sector, i.e. hiss is spread over a broader MLT range at higher By.
2. Same plot as above, but with values of IMF By for hiss events are the mean values over the past half hour from the time of hiss occurrence
shows a little difference, if any, from the previous plot. It may act to reinforce our observations.
3. Dependence of Hiss Occurrence on IMF Bz
and one with half hour averaged values of IMF Bz taken to represent Bz at hiss event time:
I am not sure if averaging Bz over past half hour is yielding some more insightful information about hiss occurrence here.
Anyways, we do observe, that as Bz decreases from positive value:
a. hiss occurrence increases in all sectors, dusk and pre-midnight.
b. hiss occurrence covers a wider range of MLT sector
c. hiss occurrence increases at nearly all frequencies for the MLT sectors where hiss is observed
4. Dependence of Hiss Occurrence on IMF Clock Angle (By and Bz):
Again taking half hour average By and Bz for hiss events puts more events in lower Bz sector:
I believe looking at the individual By and Bz variations separately gives more insight into hiss' dependence on IMF.
Substorm Onset Time & Location Dependence on IMF By & Bz:
I used the substorm onset database made using the IMAGE-FUV data taken from Frey's 2004 paper on substorm observations between May 2001 and Dec. 2002. Again, I plotted two sets of plots to see if there is a change in a general trend, first set of plots used exact values of IMF By and Bz at the time of onset, while the second set used past half hour average of IMF By and Bz from the time of substorm onset.
Note the following for each figure below:
1. There are two plots showing the dependence of Substorm Onset Geomagnetic Latitude and MLT on IMF By (or Bz) respectively.
2. Each plot has three line plots. First just plots the raw data of ~2400 onset times in two and a half year period. Second line shows the running median of the given length to smooth out the data. While the third one is an nth degree polynomial fit through this data.
3. Each plot has x-axis centered at 0 Bz, and the y-axis centered at the mean MLT (or Geom. Lat.) of the entire data.
4. Morning times have been offset by 24 to fit on the graph nicely.
1. Below is the figure showing the dependence of Substorm Onset Geomagnetic Latitude and MLT on IMF Bz for exact Bz values:
2. While figure below is the same plot with half hour averaged Bz values at each onset time:
-> MLT of Onset decreases increases a little bit from 23 to ~24 MLT for decreasing Bz. However, I can't fully explain this trend?
-> I also think that you mentioned about Substorm Onset occurring at roughly about 21MLT, which doesn't seem to be the case here?
3. I also plotted similar plots for By, and below figure is plotted using past half hour average By values at onset times:
-> MLT of onset decreases from ~24 to little below 23 with increasing By
Thus, It is difficult to say decisively here but a case can be made that with By going from -ve to +ve, Onset MLT gets closer to the dusk side and Onset latitude also gets closer to SP's geom. lat. s.t. more hiss might be seen in lower MLTs for increasing By.
IMF SuperPosed Epoch Analysis with Short and Medium Time Intervals
Here, I show the superposed epoch analysis of IMF By and Bz with respect to hiss's occurrence in 2 and 3 hr MLT intervals respectively.
1. To compare, I've re-plotted the original superposed epoch analysis with 6 hr time intervals of hiss occurrence, starting at 6 MLT. I've kept the y and x axes uniform for all figures:
2. Following figure shows the plots for 3 hr long hiss occurrence intervals, starting at 2 MLT:
We note the following observations from this figure:
First in Bz:
a. All intervals from morning up to noon have a minimum in Bz that shifts towards the 0hr mark, i.e. they need less and less time after the reversal to southward Bz for hiss to occur.
b. Also only the above times, also including the 11-14MLT interval, show a reversal from positive to negative Bz and for other intervals, Bz stays -ve throughout, getting more -ve close to the 0hr mark.
c. All the time intervals beyond noon have a minimum value in Bz that shifts away from the 0hr mark, i.e. they take more and more time before Bz sees a minima before hiss is observed.
Now, variation in By:
a. Early morning hiss, from 23-8MLT coincides with positive By, while Post-dusk to Pre-midnight hiss, from 17-23 MLT, coincides with negative By.
b. Pre-noon hiss, 8-11 MLT, corresponds to negative By, while Post-noon hiss, 11-17MLT, occurs when By is positive.
3. Below is for 2 hr long hiss occurrence intervals:
Again, similar observations can be made here as well:
From Bz plot:
a. Morning to noon hiss shows a reversal in Bz, while Bz for post-noon to midnight hiss stays negative throughout, with minimum in Bz shifting leftwards from the 0hr mark.
South Pole Magnetometer Database Read and Analysis:
I have inspected the data and have sent a set of questions to Andy regarding its formatting. Its proving to be a little difficult to read but I'll send you a few figures as soon as I read the database in.
I should first find out the baseline of the H component from the data, i.e. the mean value, right?
Then I can treat del_H as an index similar to the AE and perform occurrence rate analysis.
What other plots can I generate from this data?
