It is currently Wed Jan 19, 2022 4:00 am

 All times are UTC - 8 hours [ DST ]

 Page 1 of 1 [ 3 posts ]
 Print view Previous topic | Next topic
Author Message
 Post subject: time or timestep in GALPROPPosted: Wed Nov 02, 2011 12:43 pm

Joined: Sun Oct 16, 2011 10:53 pm
Posts: 2
I have a very basic question for GALPROP:
Does GALPROP solve the time evolution of cosmic ray momentum spectrum in the Galaxy? or just to solve for a steady state solution?

If the former is true, an important parameter should be the time. With source and loss terms, the cosmic ray (say electrons) spectrum should evolve and may be different at different times. But I could not find this parameter in the code. For example, I don't know when is the time at the final data output. I don't quite understand the parameters start_timestep and end_timestep. Are they the initial and final times of the calculation or some kind of dt? Thanks very much for your help!

Top

 Post subject: Re: time or timestep in GALPROPPosted: Wed Nov 02, 2011 1:04 pm

Joined: Wed Oct 21, 2009 6:30 pm
Posts: 47
Location: Stanford U.
Fulai, thank you for the question!

In general, GALPROP solves the time-dependent CR propagation equation. However, for most CR studies, the Galaxy is assumed to be in a steady state. In order to derive a steady state solution, GALPROP can be configured to solve the CR transport equation for a long enough period of time. With constant sources and constant losses, the solution of the CR propagation equation asymptotically approaches the steady state as time approaches infinity.

There are two methods of achieving the steady state solution:

1) The constant small time step ("CSS") solution, which makes time steps shorter than the electron energy loss time (~100 years), but makes enough of them to span the highest energy proton diffusion time (~10^9 years).

2) The "accelerated" solution, which starts with a very large time step (~10^9 years) and then gradually reduces the time step down to the smallest time scales (~10^2).

Andy Strong has made a comparison of the two methods in the Explanatory Supplement, Section 11.1 http://galprop.stanford.edu/code.php?option=manual

In order to implement the CSS solution, set, for example,
start_timestep=end_timestep=1e2
timestep_repeat=1e7
timestep_factor=0.99
(the values are just an example! the values you need to use depend on the problem you are solving).
This will make 1e7 time steps, 1e2 years each. timestep_factor may be any number less than 1.

In order to implement the accelerated solution (default in WebRun), set (for example)
start_timestep=1e9
end_timestep=1e2
timestep_repeat=100
timestep_factor=0.5
This will start with the time step equal to 1e9 years, repeat it 100 times, then set reduce the time step by 2 (i.e., multiply by timestep_factor) and repeat it 100 times, reduce it by 2 again, etc., until it is down to the time step equal to 1e2.

_________________
http://galprop.stanford.edu/

Top

 Post subject: Re: time or timestep in GALPROPPosted: Wed Nov 02, 2011 1:28 pm

Joined: Sun Oct 16, 2011 10:53 pm
Posts: 2
Thanks for the detailed explanation!

Top

 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 1 of 1 [ 3 posts ]

 All times are UTC - 8 hours [ DST ]

#### Who is online

Users browsing this forum: No registered users and 1 guest

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

Search for:
 Jump to:  Select a forum ------------------ general astrophysics    cosmic rays    gamma rays    interstellar medium    heliospheric modulation    cosmic-ray data galprop general discussion    galprop applications    galprop compilation    galprop debugging    galprop tests    galprop developer    galprop website    galprop features    galprop help    galprop WebRun    2011 GALPROP Workshop: abstracts and discussion other codes    analytical models    semi-analytical models    numerical models    comparisons with galprop    heliospheric modulation